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a
Science
The
Transactions
o f
the
British
Cave
Research
ssociation
BeR
I
o lume
15
Number 2
Augus t 9 8 8
I
Sea leve l Caves
of
Berry
Head
Devon
Caves o f
Chiapas Mexico
Aragoni te
in Daren
Cilau
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aveScience
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ave
Science
TRANSACTIONS
OF
THE
BRITISH
CAVE
RESEARCH
ASSOCIATION
Volume
15
Number
2
August
1988
Conten ts
Sea- level
Rela ted
Caves on Berry
Head
South Devon
C. J . Proctor
The
Caves
of Chiapas Southern Mexico
Terence
M
Whitaker
Aragoni te in Ogof
Daren
Ci lau
A. Kendal l
Forum
Cover:
Sunbeams in
the ent rance
of
Cueva
Borohuix
Chiapas Mexico.
By Steve Fos te r of the 1982-3 Br i t i sh Mexico Expedi t ion.
Edi tor : Dr T .D . Ford Geology Dept . Leices te r Univers i ty Leices te r LEl 7RH
39
51
83
85
Production
Editor :
Dr. A.C.
W
a l t h
am
Civ.
Eng.
Dept .
Trent
Poly techn ic
Nottingham
NGl
4BU
Cave
Science
i s published by the
Br i t i sh
Cave
Research
Assoc ia t ion
and i s
i s sued
to a l l
pa id up members of the Assoc ia t ion
1988 s ubs c r i p t i on
ra tes
a re : Individua l £10.00 I n s t i t u t i on
or
Club £12.50
Membership Secre tary:
N.
Briggs
127 Bul ls Head Lane Coventry CV3 1FW
Individua l
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Science a re obta inable from:
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Sa les 20 Woodland Avenue Westonzoy1and Bridgewater Somerset TA7
OLQ
The
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.C . R.A. i s : B.C.M.-B.C.R.A.
London WC1N 3XX.
Copyr ight the
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Research
Assoc ia t ion
1988.
No
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e d
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ISSN 0263 -7 6 0X
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CAVE SCIENCE Vol. 15, No .
2, August
1988
Transact ions of
the
Bri t i sh Cave Research
Associat ion
Sea-leve l Related
Caves
on erry
Head
South
Devon
C.
J .
PROCTOR
Abstract : Ancient marine erosion
plat forms on the Devonian
l imestone
outcrop
of
Berry
Head show that
it
has been
a
coastal
headland, surrounded
on
three sides
by the
sea,
since
Ter t i a ry
or
l o
wer Pleis tocene t imes.
Apart
from
a few small wave-eroded
sea caves,
the known caves
on
Berry Head
are exclusively phreat ic in or igin.
They
are
par t ia l ly
controlled by
Permo -Tr iass ic
extensional
f ractures and neptunean dykes,
but the
dominant
inf luence on cave
formation
on
Berry Head ha s
been the
hydrological
con tro ls dic ta ted by the unusual coastal
environment.
Horizontal cave
systems
on
the Head occur a few metres below the level of Pleis tocene
marine
erosion platforms, showing
that
they formed
dur ing sea
level
s t i l l - s t ands ,
a t
the base
of
a shallow
freshwater
lens
underlain
by
seawater .
Deep
r i f t
systems formed
a t the
lens base as
the f reshwater
lens
rose
and
f e l l
dur ing periods of
changing
sea
l eve l . The caves are
ten tat ively
dated by cor re la t ion with surface
features .
The
evidence
suggests tha t
they
formed when
sea
levels were high during i n t e rg l ac i a l
periods.
Cave development over a t leas t th ree in terg lacials i s proposed.
GEOLOGY
erry
Head l i e s a t
the
eastern
end
of
one
of
the
largest
outcrops of Devonian l imestone in
Devon. The headland
i s
surrounded
on
three sides
by the sea, and marks the
southern
extremity of
Torbay f ig . 1) . Quarrying on
the
north side of
the
Head has exposed a large
number of cave
entrances, and
exploration
since 1985 has
revealed
several hundred metres of phreat ic caves Proctor
1987), probably only a small proport ion of
the
to ta l
cave development . These caves
occupy
a
unique
posi
t
ion
in a
block
of
l imestone
almost
surrounded
by the sea, and have
an
unusual
morphology
indicat ing that
they formed a t the base
of a
th in
f reshwater
lens overlying a deep
seawater aquifer , in a manner
analogous
to
the
Blue Holes of
the Bahamas.
The following i s
thought to
be
the
f i r s t i den t i f i ca t ion of
such
a
process
having operated in
Bri ta in .
The l imestone outcrop of which Berry Head
forms the eastern end s t re tches km to
the
west to
the
shores
of the Dart
estuary
f ig . 1). The
st rat igraphy of
the
area
has
been described by
Smythe 1973)
. The Brixham
l imestones
were
deposited through most of the middle Devonian,
from
ear ly Eifel ian
t imes,
and
deposi t ion
f inal ly
ceased in
Frasnian
times early upper Devonian).
The
l imestone sequence
consis ts of
three
main
uni t s . The lowermost
uni t ,
th in c r
noidal
l imestones interbedded wi th red shales ,
i s
not
exposed
on
Berry
Head,
but
outcrops
on
the coast
to
the south.
The middle
uni t
is a
thin-bedded
c r inoidal
l imestone.
The uppermost uni t i s
composed of poorly bedded
cr inoidal
l imestones
which are l a te r a l ly replaced by massive
Figure 1 Loc a t ion ma p and g
eolo
gy
r i x h a m
~ m
K Y
Ul upp
er
lim eston e
Ml
middle limestone
Q [ J
n
ept
unean dyke
...1. L. t u It
-.E
d
ip
marine erosion
platform
raised beach
o
TOR BAY
Sho
alston
e
\
Be rry
Head
, \ , 1
1
· {
N
grid
- -
Syncli ne
H
ead
o
4
00
m
39
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stromatoporoid
- coral
ree f l imestones
on
Berry
Head i t s e l f .
The l imestone
has suffered extensive
Hercynian
deformation.
Berry Head
l i e s within
an
east-west
t rending syncline
which
brings
the
base
of
the
uppermost
reef)
l imestone
down
to below
sea
level .
Thus the main mass of the headland i s
composed
of massive
ree f l imestones
a t leas t
above present
day sea
level ) . To
the north
and
south,
middle
l imestones outcrop
on
the f lanks of
the syncline a t Shoalstone and
Durl
Head,
jus t
outs ide
the area
of
th i s
study
fig.
l ) .
The
upper
l imestone on
Berry Head
shows t igh t small
scale folding
and
well
developed
cleavages in
places . These s t ruc tures have not been mapped
s ince they appear to have r e l a t ive ly l i t t l e
influence
on cave development.
Over
most
of the
headland the l imestones dip to the south a t 10-30°
but
local ly much s teeper
dips are present.
On
Berry Head the l imestone
i s
cut by
numerous
neptunean
dykes
occupying WSW-ENE
t rending tension
fractures ,
which general ly
show
a s teep
dip (60°+)
to the south. The dyke f i l l i s
red
Permo-Triassic sandstone (usual ly ca lc i te
cemented)
with blocky calc
i t e
veins,
often
showing
evidence
of
several phases of i n f i l l .
A further
set of north-south trending ver t ical
f ractures ,
sometimes
showing
a down throw on the east s ide,
may
a l so contain
a
similar
f i l l . The
dyke
f i l l
dates
these
features to
the
Permo -
Triass ic
(Richter 1966), and they
may be at t r ibu ted to a
period of
extensional
tec ton ics connected
to
the
formation of the
English
Channel. The dykes have
been examined in
de ta i l a t
Shoals tone
Beach,
jus t
west
of Berry Head
by
Richter
(1966).
He showed
that a t Shoalstone of two generat ions of dykes are
present, an older calci te-cemented se t , and
a
younger
se t which
may
be cemented by ca lc i te or
quartz
. No such analysis of
dyke petrography
has
been
done
on
Berry
Head.
Neptunean dykes
have
considerably influenced cave formation on the
headland,
and a
study
of var ia t ions in dyke
petrography
would be par t icu lar ly valuable in
assess ing
the i r
ef fect
on cave
development .
There are extensive outcrops
of
Permo-Triassic deser t
sediments
on the floor of
Torbay
to
the north of Berry Head,
and
small
res idua l
outcrops
on
the
south
s ide.
These
outcrops
l i e
well below
the
top
of the headland,
suggest ing that
t
was a
r idge
in
Permo-Triassic
times.
The
eastern
end of
the outcrop
i s now
defined
by a major
Ter t i a ry faul t which
has
downfaulted the continuat ion of the ridge to the
east
(Laming 1982). Tert ia ry and
l a te r
erosion
has probably merely exhumed the
l imestone
ridge
from
beneath
a cover of la ter
sediments, with
l i t t l e modificat ion apart from the cut t ing
of
marine erosion platforms on i t s top and
flanks.
40
Neptunean dyke
in the roof of The Cavern: the
dyke
consists
of a wide vuggy calci te vein flanked by
narrower
veins of
sandstone
(visible
at top centre
and lower
r ight) . Dyke
width is approximately O.8m.
SURFACE
GEOMORPHOLOGY
As
mentioned above, the
gross
morphology of
the headland
was
probably determined
in
Permo-Triassic times,
when
the l imestone
outcrop
formed a s teep-s ided
r idge
towering
above
the
deser t
plain to
the
east
(Laming
1982, p 163).
Berry
Head, being composed of
massi
ve ree f
limestones, would have been par t i cu la r ly r es i s tan t
to erosion, and so formed
the
easternmost
extension of the
ridge
.
Berry Head from the
south:
the
57m
m rine
erosion pl tform
forms
the
f la t
top of the
headland, and the
minor
3m
platform can
be
seen as a ledge
jus t
above sea level
near the
end of the
head.
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There i s no
evidence
of
f luv ial
erosion on
Berry
Head
i t s e l f ,
and t i s probable
that
marine
erosion was the major process involved in the
exhumation
of
the r idge to
form
the present
coas ta l headland.
In
de ta i l the geomorphology
of
Berry Head
i s
dominated
by a
ser ies
of
marine
erosion
plat forms
and c l i f f s formed by wave
erosion
a t
sea
level
dur ing
former
sea
level
s t i l l - s tands
f igs .
1 2). These plat forms fa l l
in to two groups
dif fer ing
grea t ly
in sca le .
Major
plat forms
The
top of
the headland
i s
formed
by a
plat form a t an a l t i t ude of approximately +57m 0 . 0 .
see
appendix
for survey methods). This plat form
c lear ly
vis ib le
on the 1:25000
OS
map)
cuts
across
the folded Devonian l imestones
and,
a t
i t s
west end, ou t l i e r s
of upper Devonian s la tes . I t
extends as a 1-2kIn wide s t r i p for
Skin along
the
south coas t of Torbay
west
of Berry
Head,
and i s
backed
by h i l l s
r i s ing to an a l t i t ude of over
+120m
0 .0 .
The
plat form
has been
dissec ted
by
severa l va l leys west
of
Berry
Head,
but where
t
i s well preserved, t
varies in a l t i t ude between
+55 and +58m
0 .0 .
on the seaward s ide , sloping up
to around
+61m
0 .0 .
on the landward side. This
morphology i den t i f i es the
fea ture
as
a
marine
erosion
platform.
A submerged c l i f f - l i n e of f the
end of Berry Head shown on
Admiralty
char ts ) with
i t s base
a t
around
-40m
0 . 0 .
backs
another
platform which i s
similar
in
scale
Donovan and
St r ide 1975). The associat ion
of
t h i s
plat form
with the
submerged
c l i f f shows
that
t too i s a
marine erosion platform. These plat forms dwarf
the small erosion
plat forms
at t r ibu tab le
to shor t
sea level
s t i l l - s t a nds
dur ing the Pleis tocene i ce
ages: they must
have
been formed during
s t i l l - s tands of correspondingly
longer durat ion.
Since
sea
level
i s known
to
have f luc tua ted
almost
cont inuously dur ing the middle and upper
Pleis tocene, these fea tures are thought to be
lower Pleis tocene or Ter t i a ry
in
age.
Minor plat forms
Much smaller marine erosion plat forms have
been cut in to the
c l i f f s
of
Berry Head. These
plat forms
have
a
number
of
fea tures
in
common.
In
form
they
are
benches
genera l ly
only
a
few
metres
wide,
but
l a te r a l ly t raceable for
considerable
dis tances
along the coas t l ine .
They are
essen t i a l ly
hor izonta l
fea tures , cu t t ing across
the geological s t ruc ture , but may vary
in a l t i t ude
by up to
5m, usua l ly
sloping up
from the
seaward
to
the landward side.
These
are fea tures
charac te r i s t i c
of
marine
erosion
platforms.
Four
plat forms have
been ident i f ied f igs . 1
2) ; see
the appendix fo r survey methods.
The lowest, forming a bench vis ib le on the
Admiralty
char t ) of f the end
of
Berry Head a t
about -15m 0 . 0 . ,
i s
the
largest
of
the
minor
platforms.
Off the
end of the
headland
t i s
about 50m
wide,
but to
the north and south t
reaches a
width
of
over 100m. The
lower l imi t
of
t h i s plat form i s a t
-16m O.D.; the
upper
l imi t
i s
SOm
ICii ed
leoch
l ;
Om
OIONANCE D TUM
~
;
c
-SOm
Figure
2 Diagrammatic
prof i le of marine erosion platforms
on
Berry
Head .
41
Rai
sed beach rest ing
on t
he
+8. 5m marine
erosion platform
on
t he
north
coas t of
Berry
Head (NGR SX 9415 56 7
4),
compr i s i ng
li m
e stone
cobbles
and abundant foss i l Os tr e a in a matrix of
shelly sand.
more di f f i cu l t to ascer ta in due to
a
cover of
recent sediments, but may be as
high as
-10m
0 .0 .
The
other three plat forms are above sea
l eve l .
Jus t above high
t ide
mark i s a
narrow
general ly 5
to 15m wide) but prominent
plat form
vis ib le
in
numerous
places a l l round
the
headland f ig .
1).
The seaward
edge of
t h i s
plat form i s usua l ly a t a
height of +3
to
+3.5m O.D . but
varies between
+2.5 and +5m 0 . 0 . ) and t may slope up to
j u s t
over +6m O.D.
on the
landward side. Five
metres
higher
i s
another plat form wi
th
a
raised
beach
containing
abundant
Ostrea
and other molluscs)
res t ing
on i t .
The seaward l imi t
of
t h i s platform
l i e s a t an
a l t i t ude
of between +8.5 and +9m O.D.;
i t s upper
l imi t
cannot be ascertained due to the
s e
diment
cover .
The
plat form and
i t s
assoc ia ted
beach i s well exposed only on the nor th s ide of
Berry
Head, par t icu lar ly
between
NGR
SX
939 568
and
SX 942 568 f ig .
1) .
The
highest
of
the
minor
plat forms
i s
poor ly
exposed, and
i s
seen
clear ly
only
on
the eas t
s ide of
Oxley Head a t NGR
SX
942
562 f ig . 1) . Here
t
can
be
seen as
a
ledge
mantled
by
scree depos i t s ,
cut t ing across
folded
limestones a t an
a l t i t ude
of +28m
O.D.
The
ledge
i s halfway up a c l i f f
face,
discouraging c lose
examination, but
t
appears
to vary
l i t t l e in
a l t i t ude . Elsewhere around
Berry
Head the c l i f f s
a t t h i s a l t i t ude are
l a rge ly bur ied
beneath scree
depos i t s ,
and the
plat form
cannot
be
def in i t e ly
ident i f ied .
These plat forms are
referred
to below
by the average a l t i t udes
of
t he i r seaward s ides ,
which
i s
normally f a i r ly constant hence,
as the
-ISm, +3m, +8.5m, and +
28m platforms).
As mentionEd above, a ra i sed beach r es t s on
the +8.5m platform. Raised beaches
are known
a t
a
similar
a l t i t ude
to
the
Berry
Head
example
in
many
places
in southwest Bri tain: in
Torbay ra i sed
beaches are exposed in several loca l i t i e s res t ing
on marine erosion
plat forms
a t al t i tudes between
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+8.5 and
+10m
O.D.
Beaches of
a t l eas t two
dif feren t
ages are present ,
as demonstrated
by
the
presence a t
some
loca l i t i e s of two beaches, one
res t ing
upon
the other , for ins tance , on the north
s ide
of
Torbay
a t Hopes Nose,
NGR SX
948 634
Bowen e t a l . 1985).
The Berry Head beach
i s
par t ly
covered
by quarry spoi l and t i s not
poss ib le to see whether two beaches are present .
Bowen
e t a l . (1985)
have
shown
by
amino acid
geochronology of
molluscs in these beaches that
they
are
re fe r rable
to
s tages 5
and
7
of
the
deep
sea oxygen isotope record (Shackleton 1977). They
argue that beaches
of
a
th i rd
intermediate age are
present , though
t i s
dubious whether t he i r method
i s
su f f i c i en t ly precise to be
sure of th i s .
Unfor tunately the
Berry Head
beach f a l l s within
t h i s intermediate group, and t
i s
uncertain
whether t dates
from
isotope s tage
5 or
7.
Bowen
e t a l .
consider
on t ec tonic
groun
d s
t ha t the
platforms
on which
the
beaches
r es t are not
much
older than
the beaches. This
i s
probably cor rec t
:
the narrow
a l t i t ude
range of the plat forms impl ies
they
have not
had time
to be
s igni f icant ly
af fec ted by di f f e r en t ia l
up l i f t or
subsidence.
This
impl ies the +8. 5m plat form on
Berry Head
dates from one of the shor t sea
level
s t i l l - s tands
of the middle and upper Pleis tocene i ce ages.
The
other minor
plat forms
are
similar in
sca le , and
are
st ll well preserved,
so cannot
be
very
old.
This suggests that
they
are similar in or igin
.
Discussion
Two major
preglac ia l
plat forms are present a t
a l t i t udes
of
-40 and
+57m
O.D., demonstra t ing
t ha t
Berry Head has been
a
coastal headland
since
a t
l eas t the
lower Pleis tocene, but
more
probably
since
Ter t i a ry
t imes.
Smaller
plat forms on the
s ides of the Head a t
al t i tudes
of
- 15m, +3m, +8.5m
and
+28m
O.D. were cut
dur ing
shor t
s t i l l - s tands
in the
rapid ly
f luctuat ing
sea
levels of the
middle and
upper
Pleis tocene
i ce
ages.
The age of the
headland
i s of considerable
s igni f icance
in
consider ing
the origin of
the
caves: e i ther they are extremely
old ,
for which
there i s
no
evidence, or
they
developed
in
a
coas ta l s i tua t ion and
can be
expected
to have
been
heavily influenced by the sea and changes
in
i t s
l
ev e
l .
The
geomorphology
of the caves
demonstrates t ha t
the
l a t t e r
in terpretat ion
is
cor rec t .
GEOMORPHOLOGY OF THE CAVES
Over 50 caves, varying
in
length from 3
metres to the
215m
long Corbridge-Cavern system
are
known
on Berry
Head.
Most of the caves l i e
wi
th in
the
disused Berry Head Quarry
or
on the
adjacent coas t l ine
f ig .
3). High
grade
surveys
(Proctor
1987)
have
been
prepared of the
caves in
N
o
42
Shaky Caves a ver t i ca l system
on
a N-S fracture which has
been breached
by
quarrying.
t h i s
area
of the headland
and
t
i s upon
these
caves that
th i s study i s based. (See appendix for
survey methods).
Morphology
The
loca t ions of the
caves
discussed below
are
shown in f ig .
3 . Apart
from
a few small sea
caves, the caves in and around Berry Head Quarry
a l l
show
c lear ev i
d
ence
of
formation by phreat ic
solut ion. I r regular passage forms and passage
networks
(for instance
the
upper
l eve l s
of
Sweetwater
Pot)
that
are
typical
of
phreat ic caves
0
0
2 m
Figure 3 Sketch map of
Berry Head showing locating
the Quarry
Caves.
A -
Sweetwater
Pot
B
Corner Cave
C -
Corbridge Resurgence
Cave
D - Rift Cave.
E - Holes in
the
Wall
F The Cavern
G - Corbridge Cave
H - Shaky Caves
I -
Berry
Head
Cave
J - Hagberry Cave
K - Sta r f i sh
Cave.
L - Cutt lef ish Cave
M -
Garfish
Cave
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The
sump, Sweetwater
Po t : a v e r t i c a l
ri t
in
a
N-
S
f rac t
u re .
The
sump i s 15
metres
deep
.
are common f igs . 5
and
6) . In addi t ion a l l caves
show such charac te r i s t i c
fea tures of phrea t ic
caves as
bl ind
roof pockets
and
i r regular
solu t ion
pi t t ing on walls . Fine boxwork solu t ion textures
are
pres
e n t in some places ,
notably Sweetwater
Pot
and the
North
Wall in Corbridge Cave. A
charac te r i s t i c
and
unusual
fea ture of the caves i s
tha t even qui te la rge passages may suddenly pinch
out to i mp enet rable f i s sures . No vadose passages
are
known: the
only vadose fea tures are
the
small
t i da l
c r eeks inc ised into mud deposi ts in
Corbridge Cave.
Several sea
level phr
e a t i c caves Garf ish,
Cut t l e f i sh
and
St a r f i s h
Caves)
show en
larg
ement
of
t he i r
entrance zones by
mechanical wave act ion to
produce
sea
caves. In
Garf ish
Cave
the
en
larged
e ntranc e
passages
lead in to
unmodified
phrea t ic
passages;
Cut t l e f i sh
and St a r f i sh Caves show
phrea t ic so lu t ion textures preserved
high up
out
of reach
of
wave e r o
sion.
A
:) S.t.NDSTOHE
}
yke
i l l
=
CALCITE
o 5m
Fig
u
re
4
Cross
sec t ions
of chambers
f
ound on
a
neptunean
dy k
e ,
showing
cave
development i n i t i a t e d along ca lc i t e veins
i n the
dyke.
(A)
Th
e
Cavern. 8)
N
o r th Wall, Corbridge Ca ve.
4 3
N
o
10m
20
m O.D.
EXTENDED ELEVATION
BERRY
HEAD
CAVE
Figur
e 5
Berry
H
ea
d
Cave
,
an i so la ted
horizonta l ri t
cave.
EXTENDED ELEVATION
O D
o
10
m
SWEETWATER POT
·
0
\ ENT .
I
i
Fig ure
6
Sweetwater Pot,
a comp
lex
cave
comprisi ng
a
deep
ver t i ca l
system
formed on
N- S
f r a c tu re s . connected to
a
small
h
orizonta
l maze
a t
+25m
0 .0
.
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The dominant geological con tro ls on cave
development are the
Permo-Triassic
N S
and
WSW ENE
f rac tures . Passage development in N S f ractures
i s
seen
in caves r igh t across
the
headland
and
most of the
known
ver t i ca l cave development
i s
on
these f rac tures
f ig .
6). Of more in te r es t ,
however, i s the re la t ionship between cave
development
and
neptunean dykes
occupying
the
WSW ENE
f ractures . Neptunean dykes
are numerous
a t the west
end
of Berry Head Quarry, and
much
of
the
cave
development in th i s area
is
c lus te red
around
a large WSW ENE dyke in
the
W corner of
the quarry. This s t ruc ture has a
f i l l
of
calci te-cemented red sandstone and
blocky
ca lc i te
veins
occupying a
f i s sure or
in
places
several
para l le l f i s sures )
that
may
approach
a metre in
width. The dyke
has
a steep
southerly
dip varying
between
45°
and
ver t ica l ,
and
cave development
int imately associated
with it
can be
examined
over
a dis tance of more than 100m. Far from the dyke
being a bar r i e r to cave development, the
largest
chambers
in
Berry Head Quarry have formed along
it
The re la t ionship between the s t ruc ture of the
dyke and cave development
shows
that passage
formation
has been
i n i t i a t e d along the blocky
ca lc i te veins.
The sandstone may remain as a
barr ier
to so lu t ion , but has frequently been
44
The North Wall, Corbridge Cave,
l rge
ven formed on
neptunean dyke.
solut ion
of
limestone adjacent
to
a calc i t e
vein on the north le f t hand)
side of
the
dyke has
l e f t a
m ssive
undissolved
vein
of
s ndstone cting
s
b rrier to
solut ion
on the
south
side. The
s ndstone vein
c n
be
seen
running
from
top
centre
to
lower
r ight .
breached by
collapse
or by
solut ion
of the ca lc i te
cement.
Thus
in
the Cavern, where the dyke has a
large
cent ral
ca lc i te vein
f lanked by
narrow
veins
of
sandstone,
solut ion has caused collapse of
the
whole dyke, and
a
chamber
has
formed by
solut ion
of the l imestone on both
sides
of the dyke
f ig .
4).
In
cont ras t , a t the
North
Wall in Corbridge
Cave, the dyke consis ts of massive sandstone
with
a ca lc i te vein
along
the
north
side. Here,
the
North Wall
aven
was formed by
solut ion of
l imestone adjacent to the ca lc i te vein to the
north of the dyke, leaving a
massive
unsupported
hanging wall
of
sandstone on
the
south s ide
of the
aven
f ig . 4). The occurrence of suc h massive
cave development along th i s dyke
i s
fur ther
discussed below. Clearly the re la t ionships
between neptunean dykes
and
cave development on
Berry
Head
would
be worthy
of
more
detai led
study,
par t i cu la r ly
the
effects
of var ia t ions
in
dyke
I i
ho logy Other geological
s t ructures bedding
and Hercynian
folding)
have
exerted
a minor and
purely local inf luence on cave
development,
and
wil l
not
be discussed in th i s paper .
The
phreat ic caves
on
Berry Head can be
divided in to two
morphological
types see
f ig . 3
for
cave loca t ions) .
1) Horizontal
systems
of passages and chambers,
Corbridge
Cave,
looking down
from the base of the North Wall
aven
into
the
end
of the
horizontal
Mudfloored Chamber.
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C
o
\ NEPTUNE N DYKE
o
Sm
I
Figure 7
Cross
sect ions o f horizont l chambers showing
var ia t ions
in s ize and morphology. A) Fir s t Chamber,
Corbridge Cave.
8)
Mudfloored Chamber, Corbridge Cave.
C) Th
e Cavern . 0) Corner Cave.
which
cut
across the geologica l s t r uc t u r e s and
may
run for
a considerable
d i s t ance
a t the same
l eve l .
Passages
are genera l ly
smal l tubes
and
ri t
passages , of ten a l igned along
N-S f r ac t u r e s , and
sometimes
forming networks .
Such
hor i zonta l
passages and networks are present in many caves
ac ross the headland, bu t a re p a r t i c u la r ly well
developed
in Berry Head Cave f ig . 5) , the
upper
l eve ls
of Sweetwater Pot f ig . 6),
Hogberry
Cave
and Garf i sh Cave. Chambers may be
low
and wide,
as in Corbridge Cave, or narrower , as in Corner
Cave and the Cavern f ig .
7) .
Most of the
chambers
are formed along the
l arge
neptune
an
dyke
a t the
NW
corner
of
the quarry .
2) Deep v e r t i c a l systems of r i f t s and inc l ined
tubes, formed i n v e r t i c a l N-S f r ac t u r e s . These
a re
r es t r i c t ed
to the western p a r t of the quarry .
Sweetwater Pot f ig . 6) i s t he deepes t , with a
v er t i ca l
range
of a t l e a s t 43m:
other examples are
the Cavern Connection passages
in
Corbridge
Cave,
and the
Holes
in
the
Wall .
A
r e l a t ed fea ture i s
t he North Wall in
Corbridge
Cave,
a
l a rge aven
developed
on
a
neptunean
dyke
f ig .
4).
Both
types of passage
may be present
in the
same cave,
and
the
known v e r t i c a l caves
a l l
connect to hor i zonta l passages and networks .
As
mentioned above, the hor izonta l caves
cu t
ac ross
t he geologica l s t ru c tu re , showing t h a t they must
be
-
the
product
of
other
cont ro ls .
Cave
and eros ion
p la t fo rm
e l eva t i ons
An
extended
e leva t ion of the
quarry caves
i s
given in f igure 8 . The
surveyed
caves extend over
a v e r t i c a l range of 43m
from
-14m O.D. to 29m
O.D.)
bu t
hor i zon t a l
cave
development
i s almost
e n t i r e l y
r es t r i c t ed
to t h ree very
d i s t i n c t
a l t i tu d e ranges: -2 to +2.5m O.D.;
5 to 9m
O.D.;
and 22 to
26m
O.D.
Horizonta l
cav
e development
between
-2
and
+2.5m O.D. i s
bes t
seen along the north coas t of
the head, examples being Garf ish Cave and
Cut t l e f i sh
Cave. Cave development a t t h i s l eve l
i s
a l so
seen in Corbridge Cave and
Corbridge
Resurgence Cave, but a t both of t hese s i t e s the
or i g i na l
passages
have
been modif ied by
col l apse
and s i l t i n g .
The major development a t 5 to 9m
O.D. i s
the wide, low hor i zon t a l chambers
of
Corbridge
Cave.
These
show evidence of a complex
or i g i n , with severa l
low
chambers
a
metre or so
apar t in a l t i tu d e t hese chambers overlap one
another ,
proving the a l t i tu d e di f fe rence i s
not
merely
the product of col lapse) .
In
places
extens i ve
roof
col l apse
has occurred
notably in
Corbridge
Cave f i r s t chamber) bu t so lu t iona l
tex tures on the wal l s
and
in uncollapsed alcoves
suggest t h a t the
col l apsed chambers are no more
than
a
metre or two above
the o r ig in a l
so lu t ion
c a v i t i e s . Horizonta l passages and networks a t the
+22 to 26m O.D. l eve l
are
widespread in caves
r ig h t
ac ross the
headland.
Comparing
these
l eve ls
of hor i zon t a l cave development
with
the a l t i tu d e s
of the middle and upper Ple i s tocene marine eros ion
p la t fo rms f ig .
8)
they
a re
c le a r ly re l a t e d , with
45
Hogbe
r r y
Cave: a hor izontal r i f t passage on a N-S
f r ac tur e .
major
hor i zon t a l
cave
development
occurr ing j u s t
below each p la t fo rm t ab le 1) .
This
r e l a t i onsh i p
between
cave
development
and the marine eros ion
pla tforms
can be
fur ther
inves t iga ted by
p lo t t in g a
graph of a l t i tu d e
agains t the area of cave development. This was
done
using
the fol lowing method. The extent of
open cave passage a t one
metre
a l t i tu d e increments
est imated
by
re ference to extended
elevat ions and
survey notes) was
t raced
from
the p lan survey for
each cave.
By
laying the
t r ac i ngs
on
graph paper
the area of open passage a t
each
a l t i tu d e
increment could
be found
by count ing squares .
No
at tempt was made to co r r ec t for
poss ib l e
upward
migra t ion
of chambers
through
col l apse
since
t h i s
has been f a i r l y
minor in sca le) bu t where
sea
l eve l caves have been enlarged by wave act ion in
Garfish, Cut t l e f i sh and Sta r f i s h Caves) the
or i g i na l
passage s iz e was es t ima ted from the
extent
of remaining so lu t iona l fea tures . Graphs
of a l t i tu d e
agains t
areas of cave
development were
p l o t t ed for
indiv idual caves
f ig .
9)
and as a
cumulat ive graph of
a l t i tu d e agains t
the t o t a l
. uUtude of
marine
erosion Altitude of associated
platfonu5
hor izontal
cave6
:5
II
O.D.
- 2 to +2.5 II O.D.
+8.5 0.11.
5
to 9 m O.D.
28
II
O.D.
22
to
26
II
O.D.
Table
Correlation
of marine erosion platforms
and
horizontal caves
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~
10m
ORDN NCE
CATUM
o 30m
PASSAGE
~ C H A M '
L -
_ _ _ _
L _
a rea
o f
known cave development in
the quarry
and
adjacent coas t
( f ig . 10) .
The a l t i tu d e
/ a r e
a graph for the t o t a l
known
cave
development
( f ig . 10) shows
t h ree major
peaks
in area
o f
cave
development
a t a l t i tu d e of
-3
to
+3m O.D.; +5 to +9m O.D.;
and
+22 to +25m O.D.
This cor responds
with
the a l t i tu d e d i s t r ib u t io n of
the
major hor i zon t a l
cave development desc r ibed
above, conf i rming t h a t
major
cave
development
occurs a few metres below
each
marine eros ion
p la t fo rm ( f ig .
10
and t ab le 1) .
Examining
the
graph
( f ig
.
10)
in more
d e ta i l ,
the
major -3 to
+3m
O.D.
peak con ta in s two sma l le r
peaks in
area of cave
development, a t
-1m and
+2m
O.
D. Reference to t he
a l ti
ude/
area
graphs for
indiv idual caves
( f ig .
9) shows t h a t
the
-1m O. D.
peak
r ep re sen t s cave development in
the
uncol lapsed Garf ish , Cut t l e f i sh and
St a r f i sh
Caves, whereas
the +2m O
.D. peak represent s
cave
development
in
Corbridge
Cave and
Corbridge
Resurgence
Cave.
Both the l a t t e r caves have been
modif ied
by col l apse and s i l t i n g . The lowest
l eve ls
of
Corbridge
Cave
con ta in deposi t s
of
co l l apse deb r i s and mud
a t
l e a s t 2 to 3m t h i ck (as
shown
by
the depth
of
t i d a l
creeks
i nc i sed i n t o
the depos i t s ) , which t ogether with ex tens ive
phrea t i c
pocket ing
o f the wa l l s a t
around
Om O.D.,
suggests t h a t these passages formed a t an a l t i tu d e
of around
Om O.D
.
Thus
the a l t i tu d e of maximum
phrea t ic
cave
development
wi t h i n
the
- 3
to
+3m
O.D.
range l i e s
a t
-1 to Om O.D., 3 to 4m below
the
base of
the +3m O.D. marine e ros ion
pla tform.
ALT1TUDE
.1
0
SJE£T' Ai'DI.
CO
RNDt
POi C
VE
- - - - - - MARINE EROSION
PLATFORM
Fi gure
B
Ex t
e n
de
d e l
ev a
t ions
of
t he Be rry Head Quarry Caves ,
show i ng
the
occurrence of
extensive
horizont l passages
and large chamber s just bel ow
the l t i tudes of mar i ne erosion
platf
orm
s . Key as
i n
Figur
e 3 .
The
cave development between
+5
and +9m O.D.
i s
more complex . The a l t i tu d e /a re a graph ( f ig .
10) shows two peaks a t +6 and +8m
O.D.,
corresponding
to
the two
major
chambers of
Corbridge
Cave, which l i e a t
s l i g h t l y
d i f fe re n t
l eve l s . The
higher
of these chambers ( f i r s t
chamber)
i s
f loo red by massive col l apsed
s l abs
between
1 .5 and 2m th ick , suggest ing t h a t it
ac t ua l l y formed a t the
same
a l t i tu d e as the
other
chambers ,
a t around
+6m
O.D.
However,
as noted
above,
uncollapsed
hor i zonta l cave development i s
a l so
v i s ib le
a t
the
higher
l eve l
(+8 to
+9m O.
D.) ,
in one p lace d i r e c t l y over ly ing a chamber a t +6m
O .D.
Thus,
al though the
major
phrea t ic cave
development in
the
+5 to +9m
O.D.
range
occurs a t
around +6m O.D., 2 . 5m below
the base of the +8.5m
O.D. marine e ros ion p la t fo rm, t here i s a l so
evidence of hor i zon t a l cave development
a t
around
+8
to
+9m O.D.
This complexi ty
of cave
development may
be
r e l a t ed to
the presence in
Torbay
of
r a i s ed beaches
of a t
l e a s t
two
ages
with in
the +8.5 to +lOm O. D. a l t i tu d e range.
Cave development in
the
+22 to +2Sm O.D.
range
shows
little modif ica t ion by
co l l apse ,
and
the
maximum cave development l i e s
a t
around +24m
O .D. ( f ig . 10) , 4m below the +28m O.D. marine
e r os i on p la t fo rm.
The
a l t i tu d e of maximum
cave
development
ac t ua l l y var ies
s l i g h t ly
between
d i f fe re n t
caves
( f ig . 9) and may l i e
between +23
and
+25m
O.D.,
3
to
Sm below
the p la t fo rm.
Taking
the
evidence for
a l l
t h ree
marine
e ros ion p la t fo rms and
associa t ed l eve l s of major
l. l IHIRI DGf
DR'
t
C £S
UllGEN HE D HOCBDUl Y
CA
VE
C
VE
CAVE C
AVE
2
0 < 0 0
- 10
AREA OF
em :
DM : l O
IT
(_ . )
4 6
Fi g
ure
9 Graphs of al t i tude
against
area of
cave
dev
elop
ment
for
the
Berry
Head
Quarry
Caves .
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13/56
+30
igure 10
Graph of a l t i tude
against the tot l area of
cave
development in the Berry ead
Quarry Caves showing the
re l t i o n sh ip between the
l t i tudes of marine erosion
platforms and major
cave
development.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
~ ~
+20
ALTITUDE
metres
O.
D.
+10
~ + 8 .5- ;
o
o
-
10
cave development discussed
above, t i s c lear
that
the maximum
phreat ic
cave development occurs in a
narrow
zone
varying between 2 and 5m below the
base
of
each platform.
Discussion
The cor re la t ion
in
a l t i t ude between marine
erosion plat forms and
major
hor izonta l
cave
development on
Berry
Head shows
that
they
are
re la ted in or ig in . The caves formed a t the
same
t ime as the marine erosion platforms, and
they
formed
in
a
coastal headland surrounded on
three
s ides by
the
sea.
The coastal environment in which they formed
i s similar to tha t
of the
Blue
Holes of
the
Bahamas
Palmer
and Williams
1984)
. The Berry
Head
caves
have
morphological
s imi l a r i t i es
to
the
Blue
Holes,
which
show
the
same
fea tures of
extensive hor izonta l phrea t ic
passage networks,
and ver t i ca l phrea t ic systems developed in
ver t i ca l f rac tures .
Addi t ional
pecul ia r fea tures
charac te r i s t i c
of Blue
Holes also seen a t Berry
Head
are large
passages
and chambers which end
with no
poss ib l e continuat ion,
and solut ional
fea tur e s such as mazes , roof
pockets and
so lu t ion-pi t t ed
walls
Palmer and Heath 1985,
Palmer 1986 a) , Palmer 1986 b)) .
These
fea tures
and re la t ionships
suggest
that
the Berry Head caves formed in
a
similar
phrea t ic
environment to the Blue
Holes,
within or a t the
base
of a f reshwater lens underlain by seawater.
The poss ib le s i te s for
the formation
of hor izonta l
phr
ea t i c caves
assoc ia ted with such a
lens
are the
water
table
or the
mixing
zone
a t
the
base
of the
lens
Mylroie 1988).
On
Berry
Head,
the
relat ionships between marine erosion
plat forms
marking the
former
sea
level and
associated
hor izonta l cave development can
be
clear ly
seen.
The base of
each
marine erosion
plat form marks
the
lower l imi t
of strong
wave
act ion dur ing
i t s
formation which lay a t or below sea level) and
thus provides a measure
of
the minimum a l t i t ude of
the contemporaneous water t ab le . Horizontal cave
development
assoc ia ted
with the
plat forms on
Berry
Head
i s
a t
a maximum
in
a
narrow al t i tude
range
of
1 to 2
m
between 2 and 5m below
th
e platforms,
showing
that
solu t ion and cave formation did not
occur a t
the
water t ab le ,
but
was grea tes t a t
l eas t
2 to
5m
below.
This
suggests
that
the
hor izonta l systems formed a t the
base
of a
shallow
f reshwater
lens a
few
metres th ick . Their
associa t ion
with
the
marine erosion
plat forms
shows t ha t
as
in
the
Bahamian
Blue
Holes,
the
hor izonta l systems of
caves developed
dur ing sea
level
s t i l l - s tands.
Palmer
and
Heath
1985)
have
suggested
that
47
+3 m
20
0
300
AREA OF
CAVE
DEVELOPMENT metres
2
Altitude of marine
erosion
platform
ver t i ca l
cave systems in the
Bahamas
have formed
by
solu t ion a t the
lens
base
as the
freshwater
lens rose and
fe l l during
per iods
of changing
sea
l eve l
. Vert ica l systems
formed
in
t h i s
way
should
show
marked
variat ions in
passage
size
with
a l t i t ude , with
larger caves at a l t i tudes
where
a
slow
ra te
of
sea
level
change allowed
the
lens
base
to
remain
for l
onger. Adjacent caves should
show
s imi lar
variat ions
of
passage size
with
al t i tude, since variat ions in the ra te of sea
level
change would have af fec ted them equal ly.
The ver t i ca l r i f t systems
on
Berry Head
show
these
fea tures c lear ly . Graphs of
al t i tude
against area
of
cave
development
for the
ver t i ca l caves
f ig .
9) show
a
considerable
var ia t ion
in
area of cave
development with al t i tude for instance in
Sweetwater
Pot ,
Rif t
Cave,
the
Holes
in
the
Wall
and
Shaky
Caves), with c lear s imi l a r i t i es
between
adjacent
caves in
severa l cases
as in Sweetwater
Pot , Rif t
Cave
and the Holes
in
the Wall, which
a l l
show
an increased
area
of cave development
a t
+17 to +18m O.D.). Thus the formation of the
ver t i ca l systems can be at t r ibu ted to
solu t ion
a t
the lens
base
dur ing per iods of changing sea
level .
The
or ig in
of the
caves in
the mixing zone a t
the
base
of a f reshwater lens explains the
occurrence of i so la ted caves and passages which
close
down to narrow
f i s sures
on
Berry
Head.
Solut ion ,
and
hence
cave
formation, occurred
only
where
mixing took
place,
resu l ing in
the
formation of i so la ted
cavi t ies
interconnected
by
a
system of
narrow
f i s sures
Mylroie
1988). On ly
where
cave formation
i s
well advanced have large
explorable
systems
developed.
Over
much
of
the
headland the caves consis t of i so la ted passages
and small networks
usua l ly
formed
on N-S
f rac tures , probably
the
r esu l t of small
sca le
cave
development a t the base of
a
thin percola t ion fed
lens. Much
l a rger caves
are associated with
the
l a rge WSW-ENE t rending neptune an dyke a t the west
end
of
the
quarry,
with major hor izonta l
caves
along
the dyke,
and
deep ver t i ca l systems
developed in ver t i ca l N-S f ractures that i n t e r sec t
i t . The l a rges t chambers in the
quarry
caves are
on t h i s
dyke,
and
t he i r
morphology shows
that
the i r
formation was in i t i a ted
along
blocky ca lc i t e
veins
in the predominantly sandstone dyke. The
presence of large
caves
formed on the dyke shows
tha t because
of
the presence of ca lc i t e veins and
open f i s sures
along
i t , the
dyke
acted as a
condui t
ra ther
than a
barr ier
to
water movement.
I t
i s
probable
that
water
flow
along the
dyke
from
inland was cons tan t ly recharging the freshwater
lens in th i s area, resu l t ing in grea ter mixing and
the formation of
large
caves.
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14/56
GE
OF THE
C VES ND ASSOCIATED
M RINE
EROSION
PL TFORMS
Speleo them
depo s i t s , o f t en a s soc ia ted with
muds and othe r sediments, a re widespread in the
Berry Head caves, and a de ta i l ed
examinat ion
of
these
depos i t s coupled
wi th the
appl i ca t ion
of
spe leothem da t ing t echn iques
would
y i e l d
much
in fo rmat ion
of
re levance
in
cons ide r ing
t he i r age .
However such
s pe c i a l i s e d
methods
have
ye t to be
app l ied to the caves , and
it i s
necessa ry
to
r e ly
on
the
evidence provided
by
t he i r
geomorphology.
In
a t tempt ing
to use such
evidence
to da te
the
caves,
it i s necessa ry to
consider
the two main
morphological types hor izonta l and ve r t i c a l
caves) s e pa r a t e l y ,
s ince they
have
somewhat
d i f f e r en t or ig ins .
The hor izonta l caves a re the more amenable to
t h i s approach. They formed dur ing
sea
l eve l
s t i l l - s t a n d s of l imi ted dura t ion ;
thus
caves
a t
a n y o n e
leve l formed
over
a
l imi ted
per iod , and
can in some
cases
be
dated
by
co r r e l a t i on
wi th
marine e ros ion
p la t fo rms
and r a i s e d
beaches. In
con t r a s t
the
ve r t i c a l caves , formed
during periods
of
changing
sea l eve l , can be expected
to
have had
a much longer h i s to ry , wi
th
renewed cave
development
wi th
each new
r i s e and
f a l l
in sea
l eve l , and
d i f f e r en t
pa r t s
developing
a t
d i f f e r en t
t imes . They
w i l l
not
be f u r t he r
d iscussed
here
beyond commenting
t ha t t he i r
format ion
can be
expected to
have spanned
the pe r iod over which
hor i z on t a l
caves
have
formed on the
headland.
Of
the su r face
f ea tures which may be
co r r e l a t ed with
cave development
on Berry Head,
only the r a i s ed beaches have
been
da ted , by amino
ac id geochronology
Bowen
e t a l 1985) .
Their
ages
a re d i scussed
above:
the evidence
sugges t s
t ha t
beaches
of a t
l e a s t cwo
d i f f e r en t
ages,
r e f e r r ab l e
to oxygen i so tope s tages 5 and 7,
are
present
r e s t i ng on p la t fo rms
between +8.5
and 10m O.D. in
Torbay.
The Berry Head beach
i s
of indeterminate
age and cou ld da te from
e i t h e r
i n t e r g l ac i a l
i so tope
s tage
5 or
7) . The
p la t fo rms on
which
these
beaches l i e a re cons idered no t to be much
olde r t han
the
beaches; c la ims t ha t t hey a re much
o l de r , based
on the presence of o lde r de r ived
e r r a t i c s inco rpora ted i n to the beaches, a re
dubious.
The
o the r
marine
e ros ion p la t fo rms a re
more
prob lemat ica l , bu t l eav ing as ide fo r the
moment
the da t ing of
the
3m
O.D. p la t fo rm,
it i s
poss ib le to come to some very t en t a t i ve
conclusions
about
the
28m O.D.
platform.
t i s
gene ra l ly accep ted t ha t
during
the l a s t
i n t e r g l ac i a l i so tope s tage
5)
the sea l eve l
reached
a
high
point
of
not more t han
about
10m
O.D., a
conclusion
suppor ted by
the
evidence of
r a i s e d
beaches in
southwest
Br i t a in .
The r a i s e d
48
beach
evidence
suggests
t ha t the i so tope s tage 7
i n t e r g l ac i a l
sea
l eve l high may have reached about
the
same
a l t i t ude . I f so , then the 28m O.D.
marine
e ros ion p la t fo rm
must be
the
produc t
of
an
o lde r
high sea
l eve l .
On the above ev idence ,
it i s
poss ib le to
begin
to cons t ruc t
a
chronology
of cave
development
on
Berry
Head
over a
per iod o f
a t
l e a s t three i n t e r g l ac i a l s : t h i s i s summ7.r i s ed i n
t ab l e 2.
t
should be
remembered
t ha t t h i s
i s
a
pre l iminary model
based
on
l imi ted da ta ,
which
wi l l
probably
r equi re fu ture r evi s ion.
The
28m
O.D.
marine
e ros ion p la t fo rm
and assoc ia ted
hor izonta l caves a t +22 to 26m O.D. are
considered
to preda te the i so tope s tage 7
i n t e r g l ac i a l , and probably formed during an
e a r l i e r i n t e r g l ac i a l . This implies
an age
of
grea te r
than 250,000 years for these caves;
it
i s
impossible a t present to be more
prec i se . The
dated
r a i sed
beaches
show
t ha t the sea re tu rned to
about
the
same l eve l +8 to
10m
O.D.)
in the
s tage 5
and
7 i n t e r g l ac i a l s . The
assoc ia ted caves
between +5 and 9m O.D. most probably formed ove r
these two i n t e r g l ac i a l s , separa ted by a
co ld
per iod wi th low sea l eve l s . Cave format ion a t
t h i s
l eve l
can thus
be pr ov i s i ona l l y
da ted to two
phases;
from
245,000 to 186,000
years before
present ,
and from
122,000 to 80,000
years before
present Bowen
e t
a l 1985) .
The
cave development
between
-2
and
+2.5m O.D.
and
the
3m
O.D.
plat form) cannot a t present be
dated:
the p la t fo rm
shows
no
c l ea r r e l a t i onsh ip t o
any
dated
sediments. Thus
it
cannot
be
inco rpora ted i n to
the
chronology a t t h i s s tage . S imi la r ly
the
ea r ly
h i s to ry
of
the system cannot a t present be
specu la ted upon: the evidence of ea r ly sea l eve l
s t i l l - s t a n d s
may
have
been
masked by l a t e r cave
development.
The above
evidence suggests t ha t
the
Berry
Head caves formed dur ing i n t e r g l ac i a l per iods , a t
t imes of high sea l eve l .
During
the in te rven ing
co ld per iods , low sea l eve ls would
have
r e su l t ed
in drainage of the
caves . The
ex tens ive
spe leothem depos i t s , some
of
which
extend
below
modern
sea
l eve l , may have formed
during cold
per iods in many cases . I f caves formed
dur ing
cold per iods , t hey wi l l now
be
very deeply
submerged and
probably
inaccess ib le .
CONCLUSIONS
The phrea t i c caves
of
Berry Head formed in a
coas ta l envi ronment ,
on a
headland surrounded
on
three s ides
by the sea . Thei r morphology
and
r e l a t i ons h i ps t o marine
e ros ion p la t fo rms
shows
t ha t t hey
a re f o s s i l lens caves, formed
by
s o l u t i on in the mixing
zone
a t
the
base
of
a
Neptunean dyke t
the entrance
to
orner ve
The sandstone
dyke
has
been
breached
by
solut ion of the
calci te
cement
of the sandstone along
joints ,
leaving pendants of eroded
sandstone between the widened
jo ints .
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15/56
f reshwater
lens
a
few metres thick, and underlain
by seawater.
Sea level
s t i l l - s tands
resul ted in
the formation of hor izonta l
systems.
Vert ica l
caves
were formed dur ing
per iods
of changing
sea
l eve l , a t the lens base as
t
rose and fe l l with
the
sea
l eve l . Many
of
the
caves occur
as
i so la ted passages and
networks,
probably
formed
a t
the
base
of a lens fed by local percola t ion water.
The major cave development associa ted with the
neptunean
dyke
a t
the
west end of
the
quarry i s
be t t e r
explained as
the
r esu l t
of
enhanced mixing
a t
the lens
base due to an addi t ional input
of
freshwater
flowing
along the dyke,
suggest ing
that
these caves also acted as
a
resurgence for
percola t ion
water
from somewhat
fur ther
inland.
The hor izonta l
caves
can be
correlated
with
middle and upper Pleis tocene marine erosion
plat forms
showing t ha t t he i r formation was
contemporaneous;
the avai lable
evidence shows
that
the plat forms and
t he i r
associated
caves
formed dur ing
i n t e rg l ac i a l high
sea
level
s t i l l - s t a nds . Cave development
over
a t
l eas t
three i n t e rg l ac i a l s
i s
proposed table 2),
but the
his tory
of
the system
is
cer tain ly more complex
than th i s .
Similar
caves
may be
qui te
common
in coastal
l imestones elsewhere
in
Bri ta in , though
a t
present
only one poss ib le s i t e i s being
inves t iga ted ,
the
submarine Doolin Green
Holes
in Western I re land.
The Green Holes
are st ll
being
explored and t i s
as yet impossible to be sure of t he i r or ig in , but
Sel f (1988)
has
suggested
that
they were in i t i a ted
by freshwater solut ion, and
merely
enlarged by
l a te r
encroachment of
seawater.
This
may
explain
some impor tant di f ferences
between
the
Berry Head
caves
and
the Green
Holes;
subhorizontal
phrea t ic
networks in the Green
Holes
appear
to
be
bedding-control led and dip gent ly to the
south,
unlike the Berry Head hor izonta l
networks.
Such
di f ferences
underl ine
the need for
fur ther
work on
coas ta l caves
and
t he i r
formation.
The cor re la t ion
between
hor izonta l
caves
and
marine erosion plat forms marking Pleis tocene sea
level s t i l l - s tands suggests
some
more
in teres t ing
poss ib i l i t i e s for fur ther work.
Horizontal
caves
provide
a
potent ia l ly
valuable
too l in ident i fying
Pleis tocene sea l eve l s t i l l - s tands , since they are
more
durable than
the
eas i ly
dest royed
marine
erosion platforms. Furthermore, the close l ink
between cave formation and the
s
ea level means
t ha t the
appl ica t ion of
sediment
mapping
and
speleothem
da t ing techniques to
the caves could be
used
to provide informat ion not only
about
the
his tory
of
cave development,
but
also about
the
his tory of Quaternary sea level var iat ions in the
area.
ACKNOWLEDGEMENTS
I would l ike t o thank a l l the members of the Devon
Speleological Society who assis ted with the survey,
p ar t icu la r ly
Tim
Lee
and
Pete
Glanvi l l
,
whose surveys of
Garfish
Cave
and
Hogberry Cave
ha ve been
used
in th i s study;
T
o r
bay Borough C o u n c i l .
p a r t i c u l a r l y
N i g e l
Smal lbones ,
fo r
permission to work on Berry Head and invaluable
help
with
the
sur face surveys including the
loan of
equipment ; and Dr Peter
Smart
for
h i s comments on
t
he
manuscr ipt .
APPENDIX:
SURVEY METHODS
The study was
based
on
surveys of the Berry
Head
Quarry
Caves carr ied out
between
1985 and 1987 (Proctor 1987). The
Ple istocene
marine eros ion pla t forms
were
a l so surveyed to
determine t he i r a l t i t udes .
The
ins t ruments used were
a
Silva
type 15T
compass-clinometer and
a 30 metre
fibron tape.
The
cave entrances and
two lower level marine
erosion platforms
(at
+3 and
+8.5m O.D.)
were surveyed
to
BCRA grade
5,
using
sea
level ,
corrected
for t idal effects , as
a datum. One long
surface traverse was
necessary,
to survey
the
cave
entrances
in the west bay
of
the
guarry.
Here the grade 5
survey
was
supplemented by a
level l ing
survey
using
an automatic
level ,
to
establish
a
datum point close
to
the cave entrances.
The
caves
were
surveyed to BCRA grade 5c,
excep t
for t i da l
sea
caves, which were surveyed to
grade 2-3 using the water
level
as
a basis
for alt i tude determination. Closed
traverse
errors
on the
su
rface
surveys
were below
1%, and
on
the
underground
surveys , 2%.
Where the e r r o r s were
gre a t e r , a
resurvey was done. The c lo s ed
t raverse
error for the 600m
loop
level led to
establish a
datum
in the
west
bay
was
5mm.
The marine
erosion platform
visible halfway up the
c l i f f
on
Oxley Head
(at +28m
O.D.) was
i n c c e s ~ i b l e
and
could
not be
surveyed
using
the normal
methods.
The
height of the c l i f f
top was
fixed by
a l evel l ing
survey
(using the
OS
benchmark
49
Oxygen
i SOtope st e
Sea l .vel
Cay. denlopment.
and
e
(XlOoo
yr.)
(_ O.D.)
Pre .ta; e
7
- +
28
Horizontal cavee
between
(>250)
+22
and +26 Ill
sta . 7
-+8.5 Development
of
borbontal
(186-245)
C yee
in
+S
to
+9
_
altitude
range, early
d ....
lopunt
of
Corbridge challlber••
s t ~
5
~ 8 . 5
a.newed d ... l o ~ e n t of +5 to +911
(00-122)
cave., Corbridge can ebaabera
r.aeb t.be
1r
pnaent
extent.
UDdated
~ +3
Horizontal cave.
bet.ween
-2 and +2.5
_.
Table 2 Preliminary chronology
for
horizontal cave
developments;
ages after Bowen e t al
(1985).
on Berry
Head
coastguard sta t ion as
a
datum)
and the
alt i tude
of
the platform found
using photogrammetry.
The
a l t i tude
range of the
major
mar
ine erosion platform
forming the
top
of
Berry Head was determined fro m the
cl i f f top
levell ing
survey
mentioned above , and from the Ordnance Survey 1: 25,000 and
1 : 2 ,500 maps. Features below
sea
level
have been examined by
d iv ing bu t were not
surveyed because prec ise
a l t i t u d e s were
not reguired, and the
Torbay
Admiralty chart provided
suff ic iently
detai led
information.
ACCESS
Access
to the
caves
should
be
arranged
through
the
warden of
Berry
Head Country Park, Nigel Smallbones, at The
Bungalow, Berry Head, Brixham, Devon (telephone 0803-882619).
REFERENCES
Bowen, D.Q., Sykes, G.A., Reeves , A., Mil ler , G.H., Andrews,
J .T . ,
Brew,
J .S .
&
Ha re , P .E . ,
1985,
Amino
a c id
geochronology of ra ised
beaches in
South
West
Br i t a in .
Quaternary
Science
Reviews. 4: 279-318.
Donovan, D.T.
&
St r ide ,
A.H.,
1975, Three drowned c oa s t l i ne s
of
probable l a te Tert iary age around Devon and Cornwall .
Marine Geology. 19:M35-M40.
Laming, D.J.C., 1982, The New Red Sandstone in The Geology
of Devon". Durrance, E.M.
Laming,
D.J.C.
eds) ,
University of
Exeter.
My1roie,
J .E . , 1988, Karst of
San
Salvador. in Mylroie, J .E. ,
(ed) Field
guide to the
kars t
geology of San Salvador
I s land
.
Dept
o f
Geology,
Miss i ss ippi Sta te
Univers i ty
&
College Centre of the Finger Lakes Bahamian
f ield
s t a t i o n , San Salvador I s land , Bahamas:
17
-4 3.
Palmer,
R.
J .
&
Williams,
D.,
1984,
Cave
development under
Andros Island, Bahamas. Cave Science. 11
(1):
50-52.
Palmer , R.J . & Heath, L.M., 1985, The e f fe c t of a nc h i a l i ne
factors and
fracture
control on cave development
below
eas tern Grand Bahama. Cave
Science.
12
(3): 93-97.
Palmer, R.J. , 1986(a),
Hydrology
and speleogenesis beneath
Andros
Is land. Cave
Science.
13
(1):
7- 12.
Palmer,
R.J. ,
1986(b),
Preliminary studies of
speleogenesis
on
Cat Is land. Cave
Science
. 13
(2): 79-82.
Proctor,
C.J . 1987,
Atlas of
the
Berry
Head Caves. C J
Proctor , Radstock.
Richter, D., 1966, On the New Red Sandstone Neptunean Dykes
of the Torbay Area. Proc . Geol. Assoc . 77 (2): 173-186.
Self , C. , 1988, Sky blue pink holes. Cave Diving (COG jn l ) .
(88): 8-10.
Shackleton, N.J. ,
1977, Oxygen
isotope
s t ra t igraphy
of
the
Middle Pleistocene.
in Shotton,
F.W.
(ed),
Brit ish
Quaternary Studies. Clarendon
Press,
Oxford.
Smythe, D.K., 197 3 , Structure of the Devonian limestone
a t
Brixham.
Proc.
Ussher Soc. 2: 617-625 .
Received
August
1988
C J
Proctor
Uni versi ty
of
Bristol
Department
of
Geography
Bristol BS8 ISS
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C VE
SCIENCE
Vo l
. 1 5 ,
N o . 2 ,
August
1988
Transact ions of th e Br i t i s h Cave
Research
Associa t ion
The
Caves of Chlapas Southern Mexico
Te re nce M WHIT KER
Abstrac t : The 35 km
of
cave found
pr ior
to
1986 in Chiapas,
the
southernmost
Mexican s t a t e ,
are repor ted.
Locat ions
and
br i e f
desc
r i
pt ions
a re
given. The a c t i v i t i e s o f the Brit ish-Mexico Expedit ion
1982-
3 and th e
12.6km of
cave found is r epor ted in more
de ta i l , with
surveys and
l oca t ion
maps.
summar i o : Las 35km de cueva explorado precedentemente
de
1986 en Chiapas,
e l mas
me
r i d iona l
es tado
de Mexico, son
reportado. Las l
oca l idad
es
desc r ipc ions cor tos
son ex
pl icado.
Las
ac t iv idads de l Exp
e d i c i on
Br i tanico -
Mexico
1982 - 3 la 12.6km de cueva
descubriado, es
r epor ta
do
mas
espec i f i cado,
con
topogra f i a s ,
mapas
de loca l i dades.
Chiapas i s the southernmos t s ta te of the
United
Sta te s
of Mexico.
I t
borders the s ta t es of
Oaxaca and Veracruz in the W and
Tabasco in
the N.
Tu the E the
Rio Usumacinta
forms i t s border
with
Guatemala . I t s 74,000 sq .
km
presen t a wide var ie ty
of
kars t
environments
.
Fig
.
1) Alt i tudes
rang
e
from sea l evel
to
2900m a t
Cerro Tzontehuiz
10km NE
of San Cris tobal de Las Casas, with a corresponding
var ia t ion
in
c l ima te and vege ta t ion Fi gs . 2-5)
CLIM TE
P CI
FIC
OCE N
VEGET TION
Five major
vege ta t ion
zones
d i
s t inguished
,
depending
on a l t i t ude
and
Detai ls are in Table 3 .
can
be
r a in fa l l .
In a l l areas r a in fa l l i s plent i fu l but in some
areas
t i s markedly seasonal with
December to
Febr uary being the dryes t period . Tabl
es
1
&
2 and
f igure 5 cont ras t some
of
the var ia t ion
in
cl imate
of the areas vis i t ed . Maximum r a i n f a l l of 5000mm
occurs
in the N in the
Pichucalco
- Teapa area and to
the S where
the
Sie r ra
Madre en te
r s Guatemala. E
of
the Lagos de Montebe l l o La
Tr in i ta r i a
a t the
mountainous border with Hu eh ue te nango,
Guatemala,
about 5000mm occurs . In most of the r es t of Chiapas
r a in fa l l i s about l500mm but the Centra l Valley of
Chiapas
, conta in ing
the
Rio
Gri ja lva
Rio Grande
de Chiapas) i s much
warmer
and
dryer
than
the res t
of
Chiapas , with 800 - l000mm , and has v
i r t ua l ly
no
ra in between November
and
Apri l .
However
th e
Eastern
s l opes of the Chiapas Highlands are more
humid with some ra in in o ~ t
months
.
Annual
average
t emper a tu r es
rang
e from 13 C in the San Cris tobal '
area up to 25 - 30 C close to the Paci f ic Ocean .
Pin
e -oak fores t : corresponding
to
the Region of
San Cris toba l de
las Casas
,
Amatenango
and Teopisca
a t h
e ight s
averaging 2000m . Largely fores ted with
pines
dominant
a t higher a l t i tudes . Unwooded areas
are occup ied by meadows
and
maize f i e lds .