The Geological Story of the Isle of Wight Part 2

The land animals were still like the Wealden reptiles. Bones of large reptiles may at times be found on the sh.o.r.e at Shanklin. Several have been picked up recently. From the prevalence of cycads we may conclude that the climate of the Wealden and Lower Greensand was sub-tropical.

The existing Cycadaceae are plants of South Eastern Asia, and Australia, the Cape, and Central America. The forest of trees allied to pines and firs and cedars probably occupied the higher land.

Turtles and the corals point to warm waters. The existing species of Trigonia are Australian sh.e.l.ls. This beautiful sh.e.l.l is found plentifully in Sydney harbour. It possesses a peculiar interest, as the genus was supposed to be extinct, and was originally described from the fossil forms, and was afterwards found to be still living in Australia.

[Footnote 3: Carbonate of lime has been replaced by carbonate of iron, and the latter converted into peroxide of iron. At Sandown oxidation has gone through the whole cliff.]

[Ill.u.s.tration: FIG. 2]

COAST ATHERFIELD TO ROCKEN END

Wl _Wealden Beds._ P _Perna Bed._ A _Atherfield Clay._ Ck _Cracker Group._ Lg _Lower Gryphaea Beds._ Sc _Scaphite. "_ Lc _Lower Crioceras "_ W _Walpen Clay._ Uc _Upper Crioceras Beds._ WS _Walpen and Ladder Sands._ Ug _Upper Gryphaea Beds._ Ce _Cliff End Sands._ F _Foliated Clay._ SU _Sands of Walpen Undercliff._ Fer _Ferruginous Bands of Blackgang Chine._ B _Black Clay._ S _Sandrock and Clays._ Wh _Whale Chine._ L _Ladder Chine._ Wp _Walpen Chine._ Bg _Blackgang Chine._

Chapter V

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 pa.s.s 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 sh.o.r.e; 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 sh.o.r.e. 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 sh.o.r.e 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 ma.s.s of timber floated down an ancient river, and stranded near the mouth, just as happens with great acc.u.mulations 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 repet.i.tion 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, a.s.suming 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 sh.o.r.e 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. a.s.s._, 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 sh.e.l.l, _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 sh.o.r.e 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 sh.o.r.e, 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 ma.s.ses 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 sh.e.l.ls. 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 sh.e.l.l is spiral, and consists of several chambers, the animal living in the outer chamber, the rest being air-chambers enabling it to float. The cla.s.s 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 sh.e.l.l, either external, as in the Nautilus, or internal, as in the cuttle-fishes, the internal sh.e.l.l of which is often washed ash.o.r.e 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 sh.e.l.l 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 sh.e.l.ls of the Nautilus and its allies are of simpler form, while the Ammonites are characterised by the complicated margins of the part.i.tion walls or septa, by which the sh.e.l.ls 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 sh.o.r.e, 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 cl.u.s.ters 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 sh.e.l.ls began to take curious forms, to unwind as it were. Crioceras, a very beautiful sh.e.l.l, 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 sh.e.l.l, 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 sh.o.r.e, 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 sh.o.r.e 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 sh.o.r.e 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 sh.o.r.e 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 acc.u.mulating 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 circ.u.mference.]

[Footnote 5: _See Guide to Fossil Invertebrata_, Brit. Mus. Nat.

Hist.]

Chapter VI

THE GAULT AND UPPER GREENSAND

We have seen how the continent through which the great Wealden river flowed began to sink below the sea level, and how the waters of the sea flowed over what had been the delta of the river, laying down the beds of sandstone with some mixture of clay which we call the Lower Greensand. The next stratum we come to is a bed of dark blue clay more or less sandy, called the Gault. In the upper beds it becomes more sandy and grey in colour. These are known as the "pa.s.sage beds,"

pa.s.sing into the Upper Greensand. The thickness of the Gault clay proper varies from some 95 to 103 feet. Compared to the mainland the Gault is of small thickness in the Island, though the dark clay bands in the Sandrock mark the oncoming of similar conditions. The fine sediment forming the clay points to a further sinking of the sea bed.

In general, we find very few fossils in the Gault in the Island, though it is very fossiliferous on the mainland at Folkestone. North of Sandown Red Cliff the Gault forms a gully, down which a footpath leads to the sh.o.r.e. It is seen at the west of the Island in Compton Bay, where in the lower part some fossil sh.e.l.ls may be found.

The Upper Greensand is not very well named, as the beds only partially consist of sandstone, in great part of quite other materials. Some prefer to call the Lower Greensand Vectian, from Vectis, the old name of the Isle of Wight, and the Upper Greensand Selbornian, a name generally adopted, because it forms a marked feature of the country about Selborne in Hampshire.[6] But, though the Upper Greensand covers a less area in the Isle of Wight than the Lower, it forms some of the most characteristic scenery of the Island. One of the most striking features of the Island is the Undercliff, the undulating wooded country from Bonchurch to Niton, above the sea cliff, but under a second cliff, a vertical wall which shelters it to the North. This wall of cliff consists of Upper Greensand. In a similar way to the small undercliffs we saw at Luccombe, the Undercliff has been formed by a series of great slips, caused here by the flowing out of the Gault clay, which runs in a nearly horizontal band through the base of all the Southern Downs of the Island, the Upper Greensand lying above it breaking off in ma.s.ses, and leaving vertical walls of cliff. These walls are seen not only in the Undercliff, but also on the northern side of the downs, where they form the inland cliff overhanging a pretty belt of woodland from Shanklin to Cook's Castle, and again forming Gat Cliff above Appuldurcombe. We have records of great landslips at the two ends of the Undercliff, near Bonchurch and at Rocken End, about a century ago. But the greater part of the Undercliff was formed by landslips in very ancient times, before recorded history in this Island began. The outcrop of the Gault is marked by a line of springs on all sides of the Southern Downs. The strata above, Chalk and Upper Greensand, are porous and absorb the rainfall, which permeates through till it reaches the Gault Clay, which throws it out of the hill side in springs, some of which furnish a water supply for the surrounding towns and villages.

Where the Upper Greensand is best developed, above the Undercliff, the pa.s.sage beds are followed by 30 feet of yellow micaceous sands, with layers of nodules of a bluish-grey siliceous limestone known as Rag.

The nodules frequently contain large Ammonites and other fossils. Next follow the Sandstone and Rag beds, about 50 feet of sandstone with alternating layers of rag. The sandstones are grey in colour, weathering buff or reddish-brown, tinged more or less green by grains of glauconite. Near the top of these strata is the Freestone bed, a thick bed of a close-grained sandstone, weathering a yellowish grey, which forms a good building stone. Most of the churches and old manor and farm houses in the southern half of the Island are built of this stone. Then forming the top of the series are 24 feet of chert beds,--bands of a hard flinty rock called chert alternating with siliceous sandstone, the sandstone containing large concretions of rag in the same line of bedding. The chert beds are very hard, and where the strata are horizontal, as above the Undercliff, project like a cornice at the top of the cliff. Perhaps the finest piece of the Upper Greensand is Gore Cliff above Niton lighthouse, a great vertical wall with the cornice of dark chert strata overhanging at the top. The thickness in the Undercliff, including the Pa.s.sage Beds, is from 130 to 160 ft.

The Upper Greensand may be studied at Compton Bay, and at the Culvers; and along the sh.o.r.e west of Ventnor the lower cliff by the sea consists largely of ma.s.ses of fallen Upper Greensand, many of which show the chert strata well. In numerous walls in the south of the Island may be seen stone from the various strata--sandstone, blue limestone or rag, and also the chert.

Let us think what was happening when these beds were being formed. The sandstone is much finer than that of the Lower Greensand; and we have limestones now,--marine, not freshwater as in the Wealden. Marine limestones are formed by remains of sea creatures living at some depth in clear water. And now we come to a new material, chert. It is not unlike flint, and flint is one of the mineral forms of silica. Chert may be called an impure or sandy flint. The bands of chert appear to have been formed by an infiltration of silica into a sandstone, forming a dense flinty rock, which, however, has a dull appearance from the admixture of sand, instead of being a black semi-transparent substance like flint. But where did the silica come from? In the depths of the sea many sea creatures have skeletons and sh.e.l.ls formed of silica or flint, instead of carbonate of lime, which is the material of ordinary sh.e.l.ls and of corals. Many sponges, instead of the h.o.r.n.y skeleton we use in the washing sponge, have a skeleton formed of a network of needles of silica, often of beautiful forms.

Some marine animalcules, the Radiolaria, have skeletons of silica. And minute plants, the Diatoms, have coverings of silica, which remain like a little transparent box, when the tiny plant is dead. Now, much of the chert is full of needles, or spicules, as they are called, of sponges, and this points to the source from which some at least of the silica was derived. To form the chert much of the silica has been in some manner dissolved, and deposited again in the interstices of sandstone strata. We shall have more to say of this process when we come to speak of the origin of the flints in the chalk. Sponges usually live in clear water of some depth; so all shows that the sea was becoming deeper when these strata were being formed.

Along the sh.o.r.e of the Undercliff, Upper Greensand fossils may be found nicely weathered out. Very common is a small curved bivalve sh.e.l.l,--a kind of small oyster,--_Exogyra conica_, as are also serpulae, the tubes formed by certain marine worms. Very pretty pectens (scallop sh.e.l.ls) are found in the sandstone. Many other sh.e.l.ls, _Terebratulae_, _Trigonia_, _Panopaea_, etc., occur, and several species of ammonite and nautilus.[7] A frequent fossil is a kind of sponge, Siphonia. It has the form of an oblong bulb, supported by a long stem, with a root-like base. It is often silicified, and when broken shows bundles of tubular channels.

In the chert may often be seen pieces of white or bluish chalcedony, generally in thin plates filling cracks in the chert. This is a very pure and hard form of silica, beautifully clear and translucent.

Pebbles which the waves have worn in the direction of the plate are very pretty when polished, and go by the name of sand agates. They may sometimes be picked up on the sh.o.r.e near the Culvers.

[Footnote 6: Names proposed by the late A. J. Jukes-Browne.]

[Footnote 7: Of Ammonites, _Mortoniceras rostratum_ and _Hoplites splendens_ may be mentioned: and of Pectens, _Neithea quinquecostata_ and _quadricostata_, _Syncyclonema orbicularis_, and _aequipecten asper_.]

Chapter VII

THE CHALK

As we have traced the world's history written in the rocks we have seen an old continent gradually submerged, a deepening sea flowing over this part of the earth's surface. Now we shall find evidence of the deepening of the sea to something like an ocean depth. We are coming to the great period of the Chalk, the time when the material was made which forms the undulating downs of the south-east of England, and of which the line of white cliffs consists, which with sundry breaks half encircles our sh.o.r.es, from Flamborough Head in Yorkshire, by Dover and the Isle of Wight, to Bere in Devon. Across the Channel white cliffs of chalk face those of England, and the chalk stretches inland into the Continent. Its extent was formerly greater still.

Fragments of chalk and flint are preserved in Mull under basalt, an old lava flow, and flints from the chalk are found in more recent deposits (Boulder Clay) on the East of Scotland, pointing to a former great extension northward, which has been nearly all removed by denudation. In the Isle of Wight the chalk cliffs of Freshwater and the Culvers are the grandest features of the Island; while all the Island is dominated by the long lines of chalk downs running through it from east to west. Now what is the chalk? And how was it made? The microscope must tell us. It is found that this great ma.s.s of chalk is made up princ.i.p.ally of tiny microscopic sh.e.l.ls called Foraminifera, whole and in crushed fragments. There are plenty of foraminifera in the seas to-day; and we need not go far to find similar sh.e.l.ls. On the sh.o.r.e near Shanklin you will often see streaks of what look like tiny bits of broken sh.e.l.l washed into depressions in the sand. These, however, often consist almost entirely of complete microscopic sh.e.l.ls, some of them of great beauty. The creature that lives in one of these sh.e.l.ls is only like a drop of formless jelly, and yet around itself it forms a complex sh.e.l.l of surprising beauty. The sh.e.l.ls are pierced with a number of holes, hence their name (fr. Lat. _foramen_, a hole, and _ferre_, to bear). Through these holes the animal puts out a number of feelers like threads of jelly, and in these entangles particles of food, and draws them into itself. Now, do we anywhere to-day find these tiny sh.e.l.ls in such ma.s.ses as to build up rocks? We do. The sounding apparatus, with which we measure the depths of the sea, is so constructed as to bring up a specimen of the sea bottom.

This has been used in the Atlantic, and it is found that the really deep sea bottom, too far out for rivers and currents to bring sand and mud from the land, is covered with a white mud or ooze. And the microscope shows this to be made up of an unnumerable mult.i.tude of the tiny sh.e.l.ls of foraminifera. As the little creatures die in the sea, their sh.e.l.ls acc.u.mulate on the bottom, and in time will be pressed into a hard ma.s.s like chalk, the whole being cemented together by carbonate of lime, in the way we explained in describing the making of limestones. So we find chalk still forming at the present day. But what ages it must take to form strata of solid rock of such tiny sh.e.l.ls! And what a vast period of time it must have required to build up our chalk cliffs and downs, composed in large part of tiny microscopic sh.e.l.ls! With the foraminifera the microscope shows in the chalk a mult.i.tude of crushed fragments, largely the prisms which compose bivalve sh.e.l.ls, flakes of sh.e.l.ls of Terebratula and Rhynchonella, and minute fragments of corals and Bryozoa. Scattered in the chalk we shall also find larger sh.e.l.ls and other remains of the life of the ancient sea. The base of the cliffs and fallen blocks on the sh.o.r.e are the best places to find fossils. Much of the base of the cliffs is inaccessible except by boat. The lower strata may be examined in Sandown and Compton Bays, and the upper in Whitecliff Bay.

A watch should always be kept on the tide. The quarries along the downs are not as a rule good for collecting, as the chalk does not become so much sculptured by weathering.