10 THE NEW RAILWAY BETWEEN UPMINSTER AND ROMFORD.
spot becoming merged in a vague slope in the course of a few yards.
But, on the other hand, the position of the highest gravels towards
the northern edge of the broad plain, the very gradual descent thence
towards the present channel of the river, and the whole aspect of the
valley, is simply that of a district in which the Thames has been
slowly cutting its way laterally from north to south, and vertically
from a higher level to a lower one. The absence of well-defined
terraces is entirely caused by the softness of the strata in which
they have been cut, and occurs, as I can personally testify, wherever
a river has cut its way through soft and incoherent beds such as
London Clay. But where the same stream has been eating its way
through harder rocks, distinct terraces appear. Thus, the Geologists'
Association, in July, 1891, visited Henley-on-Thames. There, on
an eminence known as No Man's Hill, the members stood on a
gravel terrace overlying Chalk, 315 feet above Ordnance Datum, and
210 feet above the Thames at Henley, and saw, most clearly and
sharply cut, in the distance, at Remenham, another gravel terrace cut
out of the Chalk, a little more than 100 feet lower than that on which
they were standing.
Similarly in the low, drift-covered districts of Cumberland, near
the Solway, I found it impossible to map the terraces on the left
bank of the Eden below Carlisle, because they were but vaguely
indicated here and there, precisely like those of southern Essex.
But a few miles away, on the Esk at Longtown and Netherby,
two or three clearly marked terraces could be traced throughout
their course. The explanation of this difference was to be found
in the facts that, on the Esk, a soft sandstone underlying the Glacial
Drift existed to some height above the surface of the stream, and the
terraces were cut in it; while on the Eden, at the spot mentioned, the
Glacial Drift came down nearly to the water's edge. Consequently
the terraces cut in it had become as vague and doubtful, and as
impossible to trace for any distance, as those in the London Clay
of the Lower Thames. But the mapping of alluvial flats and river
terraces is part of the duty of a worker on the Geological Survey,
while it is a matter unlikely to attract the attention of most other
geologists.
The second point on which I wish to touch is this. Previous
to the discovery of Boulder Clay beneath Thames Valley Gravel at
Hornchurch, the most southerly exposures of Chalky Boulder Clay
known were those north of Romford and at Finchley. In each