Henry T. (Henry Thomas) De La Beche.

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before he described the section (1829), the remains of a row of wooden piles had been
found in this sand, sharpened for the purpose of driving, and that they appeared to
have been used in the construction of a wooden bridge for foot passengers. They
crossed the valley, and were about six feet long, their tops being about 24 feet from
the present surface, just on a level with the present low water at spring tides. He
remarks, that if the relative sea level had been then as now, such a bridge would have
been useless.

* "Report on the Geology of Cornwall, Devon, and West Somerset," p. 406 (1839).



438 FOOT-PRINTS OF DEER AND OXEN AMID THE

ground extending from the mouth of the Neath River eastward beyond
Port Talbot, fringed by a covering of blown sand-hills. Beneath these
and the low ground, natural and artificial operations have occasionally
exposed the vegetable accumulation, the stumps of trees with their
roots standing as they grew, with prostrate trunks, and the usual cha-
racteristics of the "forest." On the surface of the clay in which the
trees are rooted, foot-prints have been here and there detected, as if in
passages amid the trees by which animals found their way through
them, these foot-prints of various forms and sizes, some clearly those of
deer, while now and then a large impression would be observed resem-
bling that of some gigantic ox, having feet spreading far more widely
than any domestic ox, even of the largest size, now known. This is
not an isolated fact, for more westward (about 28 miles), while docks
were being constructed at the port of Pembre, Caermarthenshire, and
some covering sands removed, the "submarine forest," which there
occurs beneath much of the estuary of the Burry and Llwchwr was
exposed, and similar foot-prints were found, some of a great ox mingled
with those of the deer. Having attracted attention, drawings of these
impressions were made at the time. As the horns and skull of the Bos
primigenius were discovered near the same place, apparently derived
from the same beds, it may be that , the foot-prints mentioned might
have been those of this large animal.

We would thus seem to arrive at a period for the growth of these
" forests" in England, when not only species of existing -British animals
then wandered among them, but also one, if not more, of the now extinct
mammals,* leading into the times when elephants, hyaenas, and other
extinct quadrupeds also tenanted this country. Indeed, when contem-
plating from any of the adjacent heights the range of country which
includes the estuary of the Burry and Llwchwr, with its " submarine
forest," and also one of the limestone caves of that part of the country,
wherein the remains of hyaenas, rhinoceroses, and other animals are
found, the cave's mouth fronting, and not far above the range and level
of the "forest," an observer has some difficulty in very clearly sepa-
rating the time when the forest grew and the red deer of the present
time, the great extinct ox, and the rhinoceros may have ceased to be
contemporaneous, anterior to the submergence of the land beneath the
level of the adjoining ocean, in such a manner that not only the stumps
of trees remained rooted in the ground in which they grew, but the
foot-prints of mammals which roamed amid the forest of this period

* It becomes interesting, as connected with the subject, to ascertain how far any of
the localities where the antlers and bones of the Megaccros Ilibernicus are found may be
connected with the tracts of " submarine forests." The general evidence respecting
this gigantic and extinct deer would appear to be, that its remains are discovered in
fresh-water shell marls or gravels beneath existing bogs.



ROOTED TREES OF SUNK FORESTS IN SOUTH WALES. 439

also remained uninjured during the time when they "were covered over
by silt and sand.

While thus there is evidence of a change in the relative levels of sea
and land, by which the latter has been lowered several feet beneath the
former along the oceanic shores of Europe for about 20 of latitude,
there is also evidence of changes of levels on the same coasts of the
reverse kind, beaches and worn cliffs affording proofs of them, and the
remains df molluscs showing that such changes occurred after these
were of the same species as those which now inhabit the adjoining seas.
Reference has been previously made (p. 282, &c.) to the mollusc re-
mains of existing species found entombed in deposits, of the inferred
comparatively recent and very cold condition of Northern Europe ; a
time when molluscs of an Arctic character reached more southwards
than at present. Still referring to the same period, and to the evidence
pointing to a submergence and emergence of the lands of the British
Islands to the amount of 1000 to 1500 feet, and probably also of much
of Western Europe to variable depths and heights, many tracts of old
coasts and beaches would be expected, their greater or less state of
preservation depending upon local circumstances as well as on the more
general influences of different climates. Amid the varied cliffs and
beaches left by so considerable an emergence, if we are to suppose it
slow, intervals of comparative stability intervening, the observer would
anticipate much difference of level in the cliffs and beaches he may dis-
cover, expecting, nevertheless, all other circumstances being the same,
that the cliffs and beaches would be the less injured in proportion as
they were the more recent.

The coasts of Europe present many examples of cliffs and beaches
elevated above the present level of the adjoining seas, the beaches
containing fragments of the shells of molluscs still inhabiting the latter.
The coasts of the British Islands, from their position, and the variable
conditions under which they occur relatively to exposure or comparative
shelter from the Atlantic, and the variable rise and fall of tides, afford
excellent opportunities for the study of these cliffs and beaches. And
with respect to the consideration of such changes of level, the observer
should bear in mind the alterations that may b,e effected by the conver-
sion of an estuary, facing the tidal wave coming in from the Atlantic
or any other ocean, into a more spread area of water by submergence
of the land, and by converting the latter into the former by an emer-
gence, the wearing away of cliffs or the accumulation of beaches con-
tinuing up to high-water mark. For example, if the land of New
Brunswick and Nova Scotia were depressed beneath the ocean (and no
very considerable submergence would be required), so that the tidal
wave flowed freely over from the present Bay of Fundy to the Gulf of
St. Lawrence, there would be an end of the causes (p. 103) producing



440 RAISED BEACHES CONCEALED BY DETRITUS.

the very high rise of tide in that bay, and, consequently, its plane of
lines of cliffs and beaches. The same would also happen, though on a
minor scale, if the land bounding the Bristol Channel, and its continua-
tion, the Severn, was so depressed beneath its present relative level,
that the great rise of tide (46 to 50 feet) at King's Road (Bristol) and
Chepstow was no longer produced, the tidal wave sweeping onwards
without much obstruction, and passing round on the north and south of
Wales, then becoming an island. In such cases the inclined plane cor-
responding to the high-water mark would be depressed at different
depths beneath the general level. In like manner the observer should
well weigh the changes and modifications by which similar estuaries or
bays during emergence from the sea may have such tides produced in
them as are now found, so that after having cliffs worn out, or beaches
thrown up at some more equal level, these more inclined planes of the
one or the other may be formed. The modifications of the relative
heights at which cliffs and beaches may be contemporaneously formed
on all tidal coasts, according to the general level of land and sea for
the time, require very great care, as also the probable conversion of
tidal into tideless seas, and the reverse, tideless seas (employing that
term with reference to tides capable of producing very appreciable geo-
logical effects, and not strictly) affording as a whole (due reference
being made to the disturbing influences of winds) a better general level
than the high-water line on coasts variably affected by the action of
tides upon them.*

From the effects, chiefly of atmospheric influences, by which the sides
of hills and mountains are decomposed, and the disintegrated portions
descend downwards into the valleys and low grounds, as in the following
section (fig. 153), where certain rocks, 6, >, slates, for example, decom-

* It is much to be desired, that the governments of different countries having sea-
coasts would, at convenient points, ascertain the level of mean tides (not a difficult
operation), connecting the spots where this may be accomplished, as marks on the
coast itself at the actual level found may be in time obliterated from the action of the
sea or atmospheric influences, with copper bolts, or other bench-marks in, or on some
inland cliff, religious edifice, or other building likely to be preserved. By connecting
such original bench-marks, and also others inland, by a carefully considered system of
levels, not only might any variations in the relative levels of sea and land be hereafter
detected, but also movements of the like kind on the great, though tranquil scale, bo
ascertained inland, the means of obtaining the needful evidence even extending con-
siderable distances into the great continents. With this view the British Association
for the Advancement of Science had lines of level run, in 1837-8, uniting bench-marks
connected with the tides in the English Channel at Axmouth, Devon, and in the Bristol
Channel, at Porteshead, near Bristol, and at Minehead. The careful levels worked out
during the progress of the Ordnance Survey in the British Islands permit excellent
connexions with the level of mean tides around. If the various European governments
possessing sea-coasts were to establish proper tide-marks, and form connexions by
levelling between them, the relative levels of sea and land in Europe could be so ascer-
tained that any changes in it could readily be detected.



RAISED BEACHES AT PLYMOUTH.



441



posed on the surface of the hills, a, a, are more or less covered by this
detritus, accumulating in depressions, such as the valley c, many a cliff



Fig. 153.




and beach is covered up, so that inland the opportunities are less fre-
quent usually for observing them than near the sea, where a coast may
be so cut back by breakers as to exhibit the beach and cliff beneath this
kind of covering. Let, for illustration, the following section (fig. 154),



Fig. 154.




one which is not uncommon in Western England, represent a raised beach,
concealed by a covering, a, a, composed of decomposed rock and other
detritus, descending from an adjoining hill ; e, /, being the level of high
tide. Should there be a heavy modern beach at e, so that the breakers
have little access to the lower part of the modern detritus, a, a, even
the subjacent rock may be covered at that point, b; but should the
breakers act freely, so as to cut back a cliff, then neither the first
distance, 1, 1, nor the second, 2, 2, would expose the concealed beach,
the latter only showing the subjacent rock at b. When, however, the
cutting back had reached the distance, 3, 3, the beach may be well
exposed ; but should the breaker action still further wear away the cliff
to 4, 4, then no trace of the beach would be left. The subjoined section
(fig. 155) of the Hoe, Plymouth, may serve to show how this can really

Fig. 155.




happen. In it, d, d, represent the Devonian limestones of the locality,
on a part of which the beach, <?, reposes, about 30 feet above the present
high-water mark, containing the remains of shells of the same species as
are now found in the adjoining sea. At , this is covered by angular
fragments of the limestone of the hill, derived from the decomposition
of its upper part, of the same kind which fills up a cavity above at a'.
At/, the old cliff is seen behind the beach, c. This section was exposed



442



RAISED BEACHES NEAR FALMOUTII.



by blasting away the limestone rock, taken away for use in large
quantities.*

The following section of part of the Cornish coast near Falmouth
affords a useful illustration of the manner in which a raised beach may
be covered by the detritus falling over from the hill above ; in this case
over the face of an ancient cliff, which would be concealed except from
the wearing away of the coast by the breakers. The section is exposed

Fig. 156.




between Rosemullion Head and Mainporth, and the angular detritus, c,
of slate and more arenaceous beds, clearly derived from the hill, A, is
well seen to cover over the cliff, 5, and the beach, a ; in all respects
corresponding with those in the adjacent coves and bays. In this section,
the observer also finds a low level of rocks, e a, formed at the time when
the breakers, at another relative level, were cutting back the ancient
cliff, b, as similar planed portions of rocks are being now cut back on
the same coasts at a lower level. Not far distant also, on the same
coast, at a place named Nelly's Cove, the subjoined section is exposed,

Fig. 157.




wherein a, is the raised beach, 5, the supporting rock, and c the angular
deposit derived from the rocks above, and which, as it accumulated, slid
into a form corresponding with that of the beach beneath, and the old

* The section is given as seen in 1830. The raised beach was composed of pebbles of
limestone, slate, reddish porphyry (occurring in places in another part of Plymouth
Sound), and red sandstones, all rocks of the vicinity. Beneath the Plymouth Citadel,
where a sandy prolongation of this raised beach occurs, it is chiefly formed of fragments
of molluscs, of the same kinds apparently as those in the Sound adjoining. Other raised
beaches are seen on the coasts of Plymouth Sound, as under Mount Edgecumbe, at
Staddon Point, and nearly opposite the Shag Hock, on the eastern side, angular
detritus of the adjacent hills covering them all.



K A IS ED BEACH AT NEW QUAY, CORNWALL.



443



cliff behind, the covering detritus, the beach, and the supporting rocks
being all now in the process of being cut back by the heavy breakers of
the adjacent sea, which in time will obliterate all traces of the beach,
its covering, and the old cliff, leaving nothing but a bare wall of the
rocks now behind the whole.

When formed of calcareous substances, either limestone pebbles of
various sizes, or of comminuted sea-shells, raised beaches are sometimes
as highly consolidated as the rocks which may support them, carbonate
of lime thrown down under fitting conditions from a solution in water
of the bicarbonate by means of carbonic acid (p. 44) cementing the
whole together. Of the consolidation of a raised beach formed chiefly
of comminuted sea-shells, that at New Quay, on the north coast of
Cornwall, has long been celebrated. The following (fig. 158) is a section

Fig. 158.




seen on the Lookout Hill, a, a, a, being slaty and arenaceous beds
(dipping at a considerable angle) upon which the beach, 5, composed of
rounded pebbles of the adjacent rocks, cemented by consolidated sea-
shell sand, reposes. At c are layers of the same comminuted sea-shell
sand, not uncommop. on the shores and blown sandy dunes of the neigh-
bouring parts of Cornwall, the lowest layers being much consolidated,*
and being covered at d by an accumulation of angular fragments of
rocks derived from the hill above. The present level of high tide is
shown by the line e, e. In this case there would appear to have been
some modification in the condition of this part of the coast, permitting
the deposit of the layers of comminuted sea-shells after the time during
which a shingle beach was formed, and prior to the accumulation of the
covering of angular fragments ; perhaps, a time when blown sands were

* The consolidation of these sands is such that they have been long employed as build-
ing stones in the adjoining country. Much of it has been used in Crantock Church,
near New Quay, where it appears to suffer little from atmospheric influences. Ancient
stone coffins made of this modern sandstone were found in the churchyard, and one of
them was to be there seen in 1838. Consolidated sand of the like kind is to be found
in several places beneath the Cornish sandy dunes, especially when these are much
formed of comminuted sea-shells. The consolidation of part of the raised beach at New
Quay is so considerable that in breaking off a portion in which pebbles of the adjoining
rocks and of quartz are mingled with the sand, the fracture will pass sometimes through
the pebbles as well as traverse the sands and its cementing substance.



444 RAISED SEA BOTTOM, NEAR NEW QUAY.

drifted over it, as in parts of the adjacent coasts at the present day,
where such sands are driven over the shingle of ancient beaches now
removed from the action of the sea. This view is supported by a section
(fig. 159) in Eistral Bay, part (on the western side) of the projecting
land on which the other section (fig. 158) is exposed, and where slates

Fig. 159.




and more arenaceous beds, a, a,' forming a portion of the same mass with
those exhibited beneath the Look-out Hill (fig. 158, a, a, a), support
rolled pebbles, often of large size, mingled with smaller gravel and sand,
the whole constituting a kind of beach, b. This is surmounted at c by
frequent alternations of fine gravel and sand, some of the layers of the
latter being more consolidated than others. At d, the sand is less indu-
rated, and at the extremities of the dunes of the north and south become
mingled with angular fragments of rocks derived from the adjacent hills.
In this instance there would appear evidence of a portion of the sea
bottom, adjacent to the coast, having been elevated when the beach at
the Look-out Hill was uplifted.

Still keeping to the north coast of Cornwall, as it appears useful to
illustrate changes of level of this kind, where various modified eifects,
arising from them, are well exhibited in very accessible localities within
moderate distances, the observer will find good examples even of raised
sandy dunes ; thus obtaining an insight into the condition of a range of
oceanic coast, with its modifications of shingle beaches at the foot of cliffs,
shallow shores with their prolongation of blown sands, and accumulations
in shallow coast waters of the time, all upraised and variously acted upon
at the present level of breaker action. At St. Ives' and Perran Bays,
sandy dunes, accumulated when the level of the Atlantic ranged along
this land 30 or 40 feet higher than it now docs, the latter having been
since upraised, are seen perched where existing conditions could not
place them, their old supporting rocks, previously removed from breaker
action, now cut into cliffs by it. This is especially well shown in the
former bay, near Gwythian, where a cliff of hard rocks, rising 35 or 40
feet above the present high-water murk, is surmounted by a part of an
ancient beach, with old s;in<ly dunes nbove it. After this uprise, the
slope of the coast was such that on the southwest, in the direction of



RAISED DUNES OF BLOWN SAND. 445

Hayle and Lelant, conditions for the production of sandy dunes still
continued, so that in this mass of blown sands, three miles in length,
modern are partly driven over the older accumulations on the sides and
in front of the valley between Gwythian and Godrevy Head, towards
which, near Godrevy, an excellent section of a raised beach was, in
1838, to be found.

Masses of sand on coasts, acted upon by winds, and apparently not
produced by existing conditions on such coasts, have not always been
accumulated as blown sands and then elevated ; as, for example, at Porth-
dinlleyn, on the coast of Caernarvonshire, where a mass of sand covers
a clay and gravel, of the deposits termed glacial, (p. 283), and might,
at first sight, be referred to raised and sandy dunes. Careful investiga-
tion shows that this sand is an elevated sea-bottom, layers of a harder
and more argillaceous kind being interstratified with the more loose sand,
and retaining all the perforations made by marine animals when these
layers were at the bottom of the sea. Breaker action is now removing
these sands, brought within its influence by the elevation of the land,
and does not assist, with the wind, in forming sandy dunes. These
sands only constitute a portion of raised sea-bottoms, formed of either
clay, sand, and gravels, with larger blocks of rock, dispersed over the
adjoining land.

A raised beach of a very instructive kind was long since (1822) de-
scribed by Dr. Mantell,* as occurring near Brighton, where one, ele-
vated several feet above the sea, rests upon chalk, the rock of the coast,
in the same manner as the beach at Nelly's Cove, Falmouth (fig. 157),
reposes on the old slates and accompanying beds. The beach near Brigh-
ton is backed by an ancient cliff of chalk, and, above the beach, chalk
rubble, loam, &c., obscurely bedded, contain many teeth and .bones of
the fossil elephant, whence the name "Elephant Bed" has been given
it. Rolled pieces of chalk and limestone are discovered among the peb-
bles, "full of perforations made by boring shells."f In this case the
beach would appear to, have been formed prior to, or during the existence
of, the mammoth in Britain.

With regard to the fossil contents of these beaches, they afford much
information as to the exposure of the coasts of the time to differences in
the range of sea to which they may have been open ; tidal streams and
ocean currents being modified by alterations in the distribution of land
and water. Professor E. Forbes informs me that the fossil shells of the
raised beaches on the shores of the Clyde are, in many cases, those of
species which, though still living in the British seas, present a more
southern character than the molluscs now existing in them, and that

* Fossils of the South Downs, 1822.

f Mantell, "Wonders of Geology," Gth edit. (1848), p. 113, where a section, and de-
tailed description are given of the raised beach at Brighton, east of Kemp Town.



446 DISTRIBUTION OF DETRITUS IN THE ENGLISH CHANNEL,

they are confined to districts more southern and western than the Frith
of Clyde. He thence infers a change in the direction of the currents
from the south (especially in that known as Rennell's Current), this
change being probably due to the conformation of the coast lines of the
time.

It is desirable, as has been done by Mr. R. C. Austen,* to connect
these raised beaches and elevated sea-bottoms of the same geological
dates, and the submarine or sunk forests, with the present state of the
seas adjoining or covering them. After carefully considering the sub-
ject, Mr. Austen shows that although the distribution of the detritus
derived from the present coasts of France and England, in the English
Channel, and from England and Ireland on the sea-bottom to the south
of the latter, with the sediment brought down by the rivers to those
coasts, is in accordance with the arrangement which would be expected
from breaker and wind-wave action and tidal streams ; there are, espe-
cially in the central parts of the English Channel and on the outer range
of the 100 and 200 fathom soundings towards the Atlantic, bare rocks,
shingles and coarse ground, and the shells of littoral molluscs so occur-
ring as to point to the submergence of former coasts and shallow water
adjoining them. With regard to the " submarine or sunk forests," Mr.
Austen calls attention to the necessity of not limiting their extension
beneath the sea outwards to the shores where they are now discovered,
but to take a more general view of them as parts of submerged dry land;
one which would better accord with the coarse detritus at depths, or in
situations, where existing wind-wave action and tidal streams would not
transport it, and also with the remains of littoral molluscs, Patella vul-
gata,) Littorina littorea, $c., found in similar situations. The evidence ad-
duced shows very uneven ground outwards, especially towards the Atlan-
tic ; strewed over inwards by the varied detrital deposits, partly the ad-
justment of existing circumstances, partly the mixed result of these and
former conditions when the present sea-bottom was more elevated, even



Online LibraryHenry T. (Henry Thomas) De La BecheThe geological observer → online text (page 52 of 85)