BROOK AND ATHERFIELD
To most Sandown Bay is by far the most accessible place in the Island
to study the earlier strata; and for our first geological studies it
has the advantage of showing a succession of strata so tilted that we
can pass over one formation after another in the course of a short
walk. But when we have learnt the nature of geological research, and
how to read the record of the rocks, and examined the Wealden and
Greensand strata in Sandown Bay, we shall do well, if possible, to
make expeditions to Brook and Atherfield, to see the splendid
succession of Wealden and Greensand strata shown in the cliffs of the
south-west of the Island. It is a lonely stretch of coast, wild and
storm-swept in winter. But this part of the Island is full of
interest and charm to the lover of Nature and of the old-world
villages and the old churches and manor houses which fit so well into
their natural surroundings. The villages in general lie back under the
shelter of the downs some distance from the shore; a coastguard
station, a lonely farm house, or some fishermen's houses as at Brook,
forming the only habitations of man we come to along many miles of
shore. Brook Point is a spot of great interest to the geologist. Here
we come upon Wealden strata somewhat older than any in Sandown Bay.
The shore at the Point at low tide is seen to be strewn with the
trunks of fossil trees. They are of good size, some 20 ft. in length,
and from one to three feet in diameter. They are known as the Pine
Raft, and evidently form a mass of timber floated down an ancient
river, and stranded near the mouth, just as happens with great
accumulations of timber which float down the Mississippi at the
present day. The greater part of the wood has been replaced by stone,
the bark remaining as a carbonaceous substance like coal, which,
however, is quickly destroyed when exposed to the action of the waves.
The fossil trees are mostly covered with seaweed. On the trunks may
sometimes be found black shining scales of a fossil fish, _Lepidotus
Mantelli_. (A stratum full of the scales of _Lepidotus_ has been
recently exposed in the Wealden of Sandown Bay.) The strata with the
Pine Raft form the lowest visible part of the anticline. From Brook
Point the Wealden strata dip in each direction, east and west. As the
coast does not cut nearly so straight across the strata as in Sandown
Bay, we see a much longer section of the beds. On either side of the
Point are coloured marls, followed by blue shales, as at Sandown. To
the westward, however, after the shales we suddenly come to variegated
marls again, followed by a second set of shales. There was long a
question whether this repetition is due to a fault, or whether local
conditions have caused a variation in the type of the beds. The
conclusion of the Geological Survey Memoir, 1889, rather favoured the
latter view, on the ground of the great change which has taken place
in the character of the beds in so short a distance, assuming them to
be the same strata repeated. The conjecture of the existence of a
fault has, however, been confirmed; for during the last years a most
interesting section has been visible at the junction of the shales and
marls, where a fault was suspected. The shales in the cliff and on the
shore are contorted into the form of a Z. The section appears to have
become visible about 1904 (it was in the spring of that year that I
first saw it), and was described by Mr. R. W. Hooley, F.G.S. (_Proc.
Geol. Ass._, vol. xix., 1906, pp. 264, 265). It has remained visible
since.
The Wealden of Brook and the neighbouring coast is celebrated for the
number of bones of great reptiles found here, from the early days of
geological research, the '20's and '30's of last century, when
admirable early geologists, such as Dr. Buckland and Dr. Mantell, were
discovering the wonders of that ancient world, to the present time.
Various reptiles have been found besides the Iguanodon--the
Megalosaurus, a great reptile somewhat similar, but of lighter build,
with sabre-shaped teeth, with serrated edges: the Hylaeosaurus, a
smaller creature with an armour of plates on the back, and a row of
angular spines along the middle of the back; the huge _Hoplosaurus
hulkei_, probably 70 or 80 feet in length; the marine Plesiosaurus and
Ichthyosaurus, and several more; also bones of a freshwater turtle and
four types of crocodiles. In various beds a large freshwater shell,
_Unio valdensis_, occurs, and in the cliffs of Brook have been found
many cones of Cycadean plants. In bands of white sandy clay are
fragments of ferns, _Lonchopteris Mantelli_. In the shales are bands
of limestone with Cyrena, Paludina, and small oysters, and paper
shales with cyprids, as at Sandown. The shore near Atherfield Point is
covered with fallen blocks of the limestones.
The Lower Greensand is seen in Compton Bay on the northern side of the
Brook anticline. Here is a great slip of Atherfield clay. The beds
above the clay are much thinner than at Atherfield, and fossils are
comparatively scarce. On the south of the anticline the Perna bed
slopes down to the sea about 150 yards east of Atherfield Point, and
runs out to sea as a reef. Large blocks lie on the shore, where
numerous fossils may be found on the weathered surfaces. The ledges
which here run out to sea form a dangerous reef, on which many vessels
have struck. There is now a bell buoy on the reef. On the headland is
a coastguard station, and till lately there has been a sloping wooden
way from the top of the cliff to bring the lifeboat down. This was
washed away in the storms of the winter 1912-13.
Above the Perna bed lies a great thickness of Atherfield clay. Above
this lies what is called the Lower Lobster bed, a brown clay and sand,
in which are numerous nodules containing the small lobster _Meyeria
vectensis_,--known as Atherfield lobsters. Many beautiful specimens
have been obtained.
We next come to a great thickness of the Ferruginous Sands, some 500
feet. The Lower Greensand of Atherfield was exhaustively studied in
the earlier days of geology by Dr. Fitton, in the years 1824-47, and
the different strata are still referred to according to his divisions.
The lowest bed is the Crackers group about 60 ft. thick. In the lower
part are two layers of hard calcareous boulder-shaped concretions,
some a few feet long. The lower abound in fossils, and though hard
when falling from the cliffs are broken up by winter frosts, showing
the fossils they contain beautifully preserved in the softer sandy
cores of the concretions. _Gervillia sublanceolata_ is very frequent,
also _Thetironia minor_, the Ammonite _Hoplites deshayesi_, and many
more. Beneath and between the nodular masses caverns are formed, the
resounding of the waves in which has given the name of the "Crackers."
In the upper part of this group is a second lobster bed.
The most remarkable fossils in the Lower Greensand are the various
genera and species of the ammonites and their kindred. The Ammonite,
through many formations, was one of the largest, and often most
beautiful shells. There were also quite small species. The number of
species was very great. Now the whole group is extinct. They most
resembled the Pearly Nautilus, which still lives. In both the shell is
spiral, and consists of several chambers, the animal living in the
outer chamber, the rest being air-chambers enabling it to float. The
class Cephalopoda, which includes the Ammonites, the Nautilus, and
also the Cuttle-fish, is the highest division of the Mollusca. The
animals all possess heads with eyes, and tentacles around the mouth.
They nearly all possess a shell, either external, as in the Nautilus,
or internal, as in the cuttle-fishes, the internal shell of which is
often washed ashore after a rough sea. The Cephalopods are divided
into two orders. The first includes the Cuttle-fish and the Argonaut
or Paper Nautilus. Their tentacles are armed with suckers, and they
have highly-developed eyes. They secrete an inky fluid, which forms
sepia. The internal shell of extinct species of cuttle-fish, of a
cylindrical shape, with a pointed end, is a common fossil in various
strata, and is known as a Belemnite (Gr. $belemnon$ "a dart".) The
second order includes the Pearly Nautilus of the present day, and the
numerous extinct Nautiloids and Ammonoids. The tentacles of the Pearly
Nautilus have no suckers; and the eyes are of a curiously primitive
structure,--what may be called a pin-hole camera, with no lens. The
shells of the Nautilus and its allies are of simpler form, while the
Ammonites are characterised by the complicated margins of the partition
walls or septa, by which the shells are sub-divided. The chambers of
the fossil Ammonites have often been filled with crystals of rich
colours; and a polished section showing the chambers is then a most
beautiful object.[4]
Continuing along the shore, we come to the Lower Exogyra group, where
_Terebratula sella_ is found in great abundance. A reef with _Exogyra
sinuata_ runs out about 350 yards west of Whale Chine. The group is 33
ft. thick, and is followed by the Scaphites group, 50 ft. The beds
contain _Exogyra sinuata_, and a reef with clusters of Serpulae runs
out from the cliff. In the middle of the group are bands of nodules
containing _Macroscaphites gigas_. The Lower Crioceras bed (16 ft.)
follows, and crosses the bottom of Whale Chine. The Scaphites and
Crioceras are Cephalopoda, related to the Ammonites; but in this Lower
Cretaceous period a remarkable development took place; many of the
shells began to take curious forms, to unwind as it were. Crioceras, a
very beautiful shell, has the form of an Ammonite, but the whorls are
not in contact; thus making an open spiral like a ram's horn, whence
its name (Gk. $keras$, ram, $krios$, horn). Ancyloceras begins like
Crioceras, but from the last whorl continues for some length in a
straight course, then bends back again; Macroscaphites is similar, but
the whorls of the spiral part are in contact. In Scaphites, a much
smaller shell, the uncoiled part is much shorter, and its outline more
rounded. It is named from its resemblance to a boat (Gk. $skaphe$).[5]
The Walpen and Ladder Clays and Sands (about 60 ft.) contain nodules
with Exogyra and the Ammonite _Douvilleiceras martini_. The
dark-green clays of the lower part form an undercliff, on to which
Ladder Chine opens. The Upper Crioceras Group (46 ft.), like the
Lower, contains bands of Crioceras? also _Douvilleiceras martini_,
Gervillia, Trigonia, etc. It must be stated that there is some
uncertainty with regard to the ammonoids found in this neighbourhood,
Macroscaphites having been described as Ancyloceras, and also
sometimes as Crioceras. The discovery of the true Ancyloceras
(_Ancyloceras Matheronianum_) at Atherfield is described (and a figure
given) by Dr. Mantell; but what is the characteristic ammonoid of the
"Crioceras" beds requires further investigation. The neighbourhood of
Whale and Walpen Chines is of great interest. Ammonites may be found
in the bottom of Whale Chine fallen out of the rock. Red ferruginous
nodules with Ammonites lie on the shore, in the Chines, and on the
Undercliff, some of the ammonites more or less converted into
crystalline spar. Hard ledges of the Crioceras beds run into the sea.
The shore is usually covered deep with sand and small shingle; but there
are times when the sea has washed the ledges clear; and it is then that
the shore should be examined.
The Walpen and Ladder Sands (42 ft.); the Upper Exogyra Group (16
ft.); the Cliff End Sand (28 ft.); and the Foliated Clay and Sand (25
ft.), consisting of thin alternations of greenish sand and dark-blue
clay, follow. Then the Sands of Walpen Undercliff (about 100 ft.);
over which lie the Ferruginous Bands of Blackgang Chine (20 ft.). Over
these hard beds the cascade of the Chine falls. Cycads and other
vegetable remains are found in this neighbourhood. Throughout the
Atherfield Greensand fragments of the fern _Lonchopteris_
(_Weichselia_) _Mantelli_ are found. 220 ft. of dark clays and soft
white or yellow sandrock complete the Lower Greensand. In the upper
beds of the Greensand few organic remains occur. A beautiful section
of Sandrock with the junction of the Carstone is to be seen inland at
Rock above Bright-stone. The Sandrock here is brightly coloured like
the sands of Alum Bay,--though it belongs to a much older
formation,--and shows current bedding very beautifully. The junction
of the Sandrock and Carstone is also well seen in the sandpit at
Marvel.
We have now come to the end of the Lower Cretaceous, in which are
included the Wealden and the Lower Greensand. Judged by the character
of the flora and fauna, the two form one period, the main difference
being the effect of the recession of the shore line, due to the
subsidence which let in the sea over the Wealden delta, so that we
have marine strata in place of freshwater deposits. But that the
plants and animals of the Wealden age still lived in the not distant
continent is shown by the remains borne down from the land. These
strata are an example of a phenomenon often met with in geology,--that
of a great thickness of deposits all laid down in shallow water. The
Wealden of the Isle of Wight are some 700 feet thick, in Kent a good
deal thicker, the Hastings Sands, the lower part of the formation,
being below the horizon occurring in the Island: the Lower Greensand
is some 800 feet thick. In the ancient rocks of Wales, the Cambrian
and Silurian strata, are thousands of feet of deposits, mostly laid
down in fairly shallow water. In such cases there has been a
long-continued deposition of sediment, while a subsidence of the area
in which it was laid down has almost exactly kept pace with the
deposit. It is difficult not to conclude that the subsidence has been
caused by the weight of the accumulating deposit,--continuing until
some world-movement of the contracting globe has produced a
compensating elevation of the area.
[Footnote 4: Some fine ammonites may be seen at the Clarendon
Hotel, Chale,--one about 5 ft. in circumference.]
[Footnote 5: _See Guide to Fossil Invertebrata_, Brit. Mus. Nat.
Hist.]