THE STORY OF THE ISLAND RIVERS; AND HOW
THE ISLE OF WIGHT BECAME AN ISLAND
We must now consider the history of the river system of the Isle of
Wight, to which our study of the gravels has brought us. For rivers
have a history, sometimes a most interesting one, which carries us
back far into the past. Even the little rivers of the Isle of Wight
may be truly called ancient rivers. For though recent in comparison
with the ages of geological time, they are of a vast antiquity
compared with the historical periods of human history.
To understand our river systems we must go back to the time when
strata formed by deposit of sediment in the sea were upheaved above
the sea level. To take the simplest case, that of a single anticlinal
axis fading off gradually at each end, we shall have a sort of turtle
back of land emerged from the sea, as in figure 6, _aa_ being the
anticlinal axis. From this ridge streams will run down on either side
in the direction of the dip, their course being determined by some
minor folds of the strata, or difference of hardness in the surface,
or cracks formed during elevation. On each side of the dip-streams
smaller ones will flow, more or less in the direction of the strike,
and run into the main streams. Various irregularities, such as started
the flow of the streams, will favour one or another. Consider three
streams, _a_, _b_, _c_, and let us suppose the middle one the
strongest, with greatest flow of water, and cutting down its bed most
rapidly. Its side streams will become steeper and have more erosive
force, and so will eat back their courses most rapidly until they
strike the line of the streams on either side. Their steeper channels
will then offer the best way for the upper waters of the streams they
have cut to reach the sea; and these streams will consequently be
tapped, and their head waters cut off to flow to the channel of the
centre stream. We shall thus have for a second stage in the history a
system such as is shown in fig. 7. The same process will continue till
one river has tapped several others; and there will result the usual
figure of a river and its tributaries, to which we are accustomed on
our maps. We shall observe that tributaries do not as a rule gradually
approach the central stream, but suddenly turn off at nearly a right
angle from the direction in which they are flowing, and, after a
longer or shorter course, join at another sharp angle a river flowing
more or less parallel to their original direction.
[Illustration: FIG. 6]
[Illustration: FIG. 7]
DEVELOPMENT OF RIVER SYSTEMS
The Chalk and overlying Tertiary strata were uplifted from the sea in
great folds forming a series of such turtle-backs as we have been
considering. The line of upheaval was not south-west and north-east,
as that which raised the older formations in bands across England, but
took place in an east and west direction. The main upheaval was that
of the great Wealden anticline. Other folds produced the Sandown and
Brook anticlines, and that of the Portsdown Hills. The upheaval seemed
to have been caused by pressure acting from the south, for the steeper
slope of each fold is on the northern side. Our latest Oligocene
strata are tilted with the chalk, showing that the upheaval took place
after Oligocene times. But the great movement was in the main earlier
than the Pliocene. For on the North Downs near Lenham is a patch of
Lower Pliocene deposit resting directly on the Chalk, the older
Tertiary strata having been removed by denudation, clearly due to the
uplift of the Wealden anticline. The raising of the Pliocene deposit
to its present position proves that the same movement was continued at
a later time, probably during the Pleistocene. But the greater part of
the movement may be assigned to the Miocene, the period of great
world-movements which raised the Alps and the Himalaya.
Many remarkable, and, at first sight, very puzzling features connected
with the courses of rivers find an explanation when we study the river
history. Thus, looking at the Weald of Kent and Sussex, we see that it
consists of comparatively low ground rising to a line of heights east
and west along the centre, and surrounded on all sides but the
south-east by a wall of Chalk downs. If we considered the subject, we
should suppose that the drainage of the country would be towards the
south-east, which is open to the sea. Not so. All the rivers flow from
the central heights north and south,--go straight for the walls of
chalk downs, and cut through the escarpment in deep clefts to flow
into the Thames and the Channel. This is explained when we remember
that the rivers began to flow when the great curve of strata rose
above the sea. Though eroded by the sea during its elevation, yet when
it rose above the waters the arch of chalk must have been continuous
from what are now North Downs to South. And from the centre line of
the great turtle back the streams began to flow north and south,
cutting in the course of ages deep channels for themselves. The
greater erosion in their higher courses has cut away the mass of chalk
from the centre of the Weald, but the rivers still flow in the
direction determined when the arch was still entire.
We have a similar state of things in the Isle of Wight. Any one not
knowing the geological story, and looking at the geography of the
Island, might naturally suppose that there would be a stream flowing
from west to east, through the low ground between the two ranges of
downs, and finding its way into the sea in Sandown Bay. Instead of
this the three rivers of the Island, the two Yars and the Medina, all
flow north, and cut through the chalk escarpment of the Central downs,
as if an earthquake had made rifts for them to pass, and so find their
way into the Solent. The explanation is the same as in the case of the
Weald. The rivers began to flow when the Chalk strata were continuous
over the centre of the Island; and their course was determined when
the east and west anticlinal axis rose above the sea.
We shall notice, however, that the Island rivers start from south of
the anticlinal axis. The centre of the Sandown anticline runs just
north of Sandown, but the various branches of the Yar and Medina flow
from well south of this. The explanation would appear to be that the
anticline is almost a monoclinal curve,--that is to say, one slope is
steep, the other not far from horizontal. Streams starting from the
ridge would flow with much greater force down the northern than the
southern side, and would cut back their course much more quickly.
Thus they would continually cut into the heads of the southern
streams, and turn the water supplying them into their own channels.
In its early history a river cuts out its bed, and carries along
pebbles, sand and mud to the sea. The head waters are constantly
cutting back, and the slope becoming less steep, till a time comes
when the stream in its gently inclined lower course has no more power
to excavate, and the finer sediment, which is all that now reaches the
lower river, begins to fill up the old channel. And so the alluvium is
formed which fills the lower portions of our river valleys.
Beyond this, the great rush of waters from melting snows and ice of
the Glacial Period has come to an end. The gentler and diminished
streams of a drier age have no power to roll flint stones along and
form beds of gravel. Gravel terraces border our river valleys at a
higher level than the present streams. Periods alternated during which
gravels were laid down by the river, and when the river acquiring more
erosive force, by an elevation of the land giving its bed a steeper
gradient, or a wetter climate producing a greater rush of water, cut a
new channel deeper in the old valley. So our valleys in Southern
England are frequently bordered by a succession of gravel terraces,
the higher ones being the older, dating from times when the river
flowed at a higher level than at present. Such terraces may be seen
above the Eastern Yar and its tributary streams. In the centre of the
old gravels is the alluvial flat of a later age.
The Island rivers cut out their channels when the land stood at a
higher level than at present. The old channels of the lower parts of
the rivers are now filled with alluvium, partly brought down by the
rivers and partly marine. The channels are cut down considerably below
sea level; and by the sinking of the land the sea has flowed in, and
the last parts of the river courses are now tidal estuaries. The sea
does not cut out estuaries. They are the submerged ends of river
valleys.
Some idea may be formed of the antiquity of our Island rivers by
observing the depth of the clefts they have cut through the downs at
Brading, Newport, and Freshwater. But to this we must add the depth at
which the old channels lie below the alluvium. It would be interesting
to know the thickness of the alluvium. But it is not often that
borings come to be made in river alluvia. However, in the old Spithead
forts artesian wells are sunk; and these pass through 70 to 90 feet of
recent deposits before entering Eocene strata. Under St. Helen's Fort,
at the mouth of Brading Harbour, are 80 feet of recent deposits. The
old channel of the Yar, at its mouth, must lie at least at this depth.
Before it passes through the gap in the Chalk downs the Yar has
meandered about, and formed the alluvial flat called Morton marshes.
These marshes stretch out into the flat known as Sandown Level, which
occupies the shore of the bay between Sandown and the Granite Fort.
What is the meaning of this extension of the alluvium away from the
course of the river out to the sea at Sandown? A glance at it as
pictured on a geological map will suggest the answer. We see clearly
the alluvia of two streams converging from right and left, and uniting
to pass to the sea through Brading Harbour. But the stream to the
right has been cut off by the sea encroaching on Sandown Bay: only the
last mile of alluvium is left to tell of a river passed away. We must
reconstruct the past. We see the Bay covered by land sloping up to
east and south east, the lines of downs extending eastward from
Dunnose and the Culvers, and an old river flowing northward, and
cutting through the chalk at Brading after being joined by a branch
from the west. This old river must have been the main stream. For it
was a transverse stream, flowing nearly at right angles to the ridge
of the anticline; while the Yar comes in as a tributary in the
direction of the strike. Of other tributary streams, all from the
right are gone by the destruction of the old land. On the left streams
would flow in from the combes at Shanklin and Luccombe--streams which
have now cut out Shanklin and Luccombe chines.
Passing the gap in the downs the river meandered about, and, with
marine deposit, washed in by the tides, formed the expanse of alluvium
which occupies what was Brading Harbour,--a harbour which in old times
presented at high tide a beautiful spectacle of land-locked water
extending up to Brading. Inclosures and drainings have been made from
time to time, the upper part near Yarbridge being taken in in the time
of Edward I. Further innings were made in the reign of Queen
Elizabeth; and Sir Hugh Middleton, who brought the New River to
London, made an attempt to enclose the whole, but the sea broke
through his embankment. The harbour was finally reclaimed at great
cost in 1880, the present embankment enclosing an area of 600 acres.
The history of the Western Yar is similar to that of the Eastern. The
main stream must have flowed from land now destroyed by the sea
stretching far south of Freshwater Gate. All that is left is its tidal
estuary, and the gravel terraces and alluvial flat formed in the last
part of its course. Of a tributary stream an interesting relic
remains. For more than 2 miles from Chilton Chine through Brook to
Compton Grange a bed of river gravel lies at the top of the cliff,
marking the course of an old stream, of which coast erosion has made a
longitudinal section. This was a tributary of the Yar, when the
mammoth left his remains in the gravel at Grange Chine and Freshwater
Gate. Down the centre of the gravels lies a strip of alluvium laid
down by a stream following the same course in later days. The sea had
probably by this time cut into the stream; and it most likely flowed
into the sea somewhere west of Brook. In the alluvium hazel nuts and
twigs of trees are found at Shippard's Chine near Brook.
The lower course of the Medina is a submerged river valley, the tide
flowing up to Newport. The river rises near Chale, and flows through a
strip of alluvium, overgrown with marsh vegetation, known as "The
Wilderness." This upper course of the Medina, from the absence of
gravels or brick earth, has the appearance of a comparatively modern
river. But the Medina has a further history. If you look at the map
you will see branches of the Yar running south to north as transverse
streams, but the main course is that of a lateral river. Look at the
two chief sources of the Yar--the stream from near Whitwell and Niton,
and that from the Wroxall valley. When they get down to the marshes
near Rookley and Merston, they are not flowing at all in the direction
of Sandown or Brading. They rather look as if they would flow along
the marshy flat by Blackwater into the Medina. But the Yar cuts right
across their course, and carries them off eastward to Sandown. When we
look, we find a line of river valley with a strip of alluvium running
up from the Medina at Blackwater in the direction of these two
streams--a valley which the railway up the Yar valley from Sandown
makes use of to get to Newport. There can be little doubt that these
streams from Niton and Wroxall originally ran along this line into the
Medina; but the Yar, cutting its course backward, has captured them,
and diverted their course. They probably represent the main branches
of the Medina in earlier times, the direction of flow from south-east
to north-west instead of south to north being possibly due to the
overlapping in the neighbourhood of Newport of the ends of the Brook
and Sandown anticlines. The sheet of gravel on Blake Down belongs to
this period of the river's history. The river must have diverted
between the deposition of the Plateau Gravels and that of the Valley
Gravels of the Yar. For the former follow the original valley, the
latter the new course of the river.
We must now take a wider outlook, and see what became of our rivers
after they had flowed across what is now the Isle of Wight from south
to north. We have been speaking of times when the Island was of much
greater extent than at present. Standing on the down above the
Needles, and looking westward, we see on a clear day the Isle of
Purbeck lying opposite, and we can see that the headland there is
formed by white chalk cliffs like those beneath us. In front of them
stand the Old Harry Rocks, answering to the Needles, both relics of a
former extension of the land. In fact Purbeck is just like a
continuation of the Isle of Wight. South of the Chalk lie Greensand
and Wealden strata in Swanage Bay, and north towards Poole are
Tertiaries. Clearly these strata were once continuous with those of
the Isle of Wight. We must imagine the chalk downs of the Island
continued as a long range across what is now sea, and on through
Purbeck. A great Valley must have stretched from west to east, north
of this line, along the course of the Frome, which runs through
Dorset, and now enters the sea at Poole Harbour, on by Bournemouth,
and along the present Solent Channel--a valley still much above sea
level, not yet cut down by rivers and the sea--and down the centre of
this valley a river must have flowed, which may be called the River
Solent. It received as tributaries from the south the rivers of the
Isle of Wight, and others from land since destroyed by the sea. There
flowed into it from the north the waters of the Stour and Avon, and an
old river which flowed down the line of what is now Southampton Water.
Southampton Water looks like the valley of a large river, much larger
than the present Test and Itchen. Its direction points to a river from
the north west; and it has been shown by Mr. Clement Reid that the
Salisbury rivers--Avon, Nadder, and Wily--at a former time, when they
flowed far above their present level--continued their course into the
valley of Southampton Water. For fragments of Purbeck rocks from the
Vale of Wardour, west of Salisbury, have been found by him in gravels
on high land near Bramshaw, carried right over the deep vale of the
Avon in the direction of the Water. The lower Avon would originally be
a tributary of the Solent River; and it enters the sea about mid-way
between the Needles and the chalk cliffs of Purbeck, just opposite the
point where we might suppose the sea would have first broken through
the line of chalk downs. No doubt it broke through a gap made by the
course of an old river from the south, as it is now breaking through
the gap made by the old Yar at Freshwater. When the river Solent had
been tapped at this point, the Avon just opposite would have acquired
a much steeper flow, causing it to cut back at a faster rate, till it
cut the course of the old river which ran by Salisbury to Southampton,
and, having a steeper fall, diverted the upper waters of this river
into its own channel.
[Illustration: FIG. 8
THE OLD SOLENT RIVER]
Frost and rain and rivers cut down the valleys of the river system for
hundreds of feet; the sea which had broken through the chalk range
gradually cut away the south side of the main river valley from Purbeck
to the Needles; and eventually the valley itself was submerged by a
subsidence of the land, and the sea flowed between the Isle of Wight
and the mainland.
A gravel of somewhat different character to the rest is the sheet of
flint shingle at Bembridge Foreland. It forms a cliff of gravel about
25 feet high resting on Bembridge marls, and consists of large flints,
with lines of smaller flints and sand showing current bedding, and also
contains Greensand chert and sandstone, which must have been brought
from some district beyond the Chalk. The shingle slopes to north-east.
To the south-west it ends abruptly, the dividing line between shingle
and marls running up steeply into the cliff. This evidently marks an
old sea cliff in the marls, against which the gravel has been laid
down.[16]
One or two comparatively recent deposits may be mentioned here. At the
top of the cliff in Totland Bay, about 60 ft. above the sea, for a
distance of 350 yards, is a lacustrine deposit, consisting in the main
of a calcareous tufa deposited by springs flowing from the limestone of
Headon Hill. The tufa contains black lines from vegetable matter, and
numerous land and freshwater shells of present-day species--many species
of Helix, especially H. nemoralis and H. rotundata, Cyclostoma elegans,
Limnaea palustris, Pupa, Clausilia, Cyclas, and others.
On the top of Gore Cliff is a deposit of hard calcareous mud, reaching
a thickness of about 9 feet, and forming a small vertical cliff above
the slopes of chalk marl. It extends north a few yards beyond the
chalk marl on to Lower Greensand. It has been formed by rainwash from
a hill of chalk, which must once have existed to the south. The
deposit contains numerous existing land-shells, especially _Helix
nemoralis_ and other species of Helix.
Between Atherfield and Chale at the top of the cliff is a large area
of Blown Sand. The sand is blown up from the face of the cliff below.
It reaches a thickness of 20 feet, and possibly more in places, and
forms a line of sand dunes along the edge of the cliff. The upper part
of Ladder Chine shows an interesting example of wind-erosion. The sand
driven round it by the wind has worn it into a semi-circular hollow
like a Roman theatre.
Small spits, consisting partly of blown sand, extend opposite the
mouths of the Western Yar, the Newtown river, and the most
extensive--at the mouth of the old Brading Harbour, separating the
present reduced Bembridge Harbour from the sea. This is called St.
Helen's Spit, or "Dover,"--the local name for these sand spits.
[Footnote 16: Fig. 9, p. 79.]