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 "passage beds,"
passing 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 shore. It is seen at the west of the Island in Compton
Bay, where in the lower part some fossil shells 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 masses, 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
passage 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 Passage Beds, is from 130
to 160 ft.
The Upper Greensand may be studied at Compton Bay, and at the Culvers;
and along the shore west of Ventnor the lower cliff by the sea
consists largely of masses 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 shells formed
of silica or flint, instead of carbonate of lime, which is the
material of ordinary shells and of corals. Many sponges, instead of
the horny 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 shore of the Undercliff, Upper Greensand fossils may be
found nicely weathered out. Very common is a small curved bivalve
shell,--a kind of small oyster,--_Exogyra conica_, as are also
serpulae, the tubes formed by certain marine worms. Very pretty pectens
(scallop shells) are found in the sandstone. Many other shells,
_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 shore 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_.]