164 A. J. JUKES-BROWNE ON SOME POSTGLACIAL RAVINES 
The pre-existing valleys seem to have 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 Boulder- 
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 N.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 
débris. Fig. 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. 
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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 les to have been coexistent with the main drift-filled 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 valley from its 
tributary. 
Still impeded by the mounds of drift in the old valleys, the modern 
beck has made a second cut through the low Chalk hills which jut 
