File:The Quarterly journal of the Geological Society of London (14052672156).jpg

164
A. J. JUKES-liROWIS^E ON SOME POSTGLACIAL EAYINES
The pre-existing valleys seem to ha^e been filled 'with drift forced
into them from the north or north-east ; that to the south of Hatcliffe
is blocked with immense mounds of- gravel, sand, loam, and Bonlder-
clay, so mingled that it is impossible to separate them completely on
the Survey map. These glacial deposits form a broad tract filling"
the ancient valley and extending to within half a mile of Thorganby,
where they end in a sort of natural ampitheatre formed by the
closing in of the chalk hills.
Through a narrow gap in these hills the modern stream makes
its way, and thence flows northward for about half a mile over a
Boulder-clay flat, till it suddenly bends JN'.W. and enters a mag-
nificent ravine excavated through the hill which forms the left
flank of the ancient valley. The slopes of this ravine are, as usual,
planted with fir trees, and the whole scene, when viewed from one
of the curves in the valley, presents quite an Alpine appearance.
The depth of the gorge about its central part cannot be less than
100 feet, and in one place on the outside of the last curve, where the
stream would act with the greatest force, a vertical cliff appears
to have existed, the upper part of this still remaining as a bare face
of chalk, while its foot is buried in a long talus-slope of fallen
debris. Tig. 4 is a section across this part of the ravine, and
though drawn from an estimate by eye only, and not from actual
measurement, it may be taken as being a fairly accurate view as
regards width and angle of slope.
Fig. 4. — Section across ravine S. W. of Hatcliffe.
Emerging from this ravine, the stream enters a much broader valley
just below a great mound of Boulder-clay which has blocked up the
drainage and caused the formation of a small lake called Croxby Pond.
The occurrence of this Boulder- clay proves the valley in which the
lake lies to have been coexistent with the main drift-fiUed valley to
the eastward, and a little lower down a hollow filled with Boulder-
clay actually leads from one valley to the other ; there can be little
doubt therefore that the Croxby valley was tributary to the ancient
valley south of Hatcliffe, and that the modern stream has made a cut
through the hill which originally separated the main vaUey from its
tributary.
Still impeded by the mounds of drift in the old valleys, the modern