Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/156

118 118 S. V. WOOD, JUN., AND F. W. HARMER ON THE Over the area thus denned, it seems to us, the ice rested during the accumulation of the Middle Glacial, and so kept that formation out of it ; and the cause (whatever it was) which put an end to the formation of sand and gravel, at least in the neighbourhood of the land-ice, was coincident with the commencement of a reces- sion in that ice. As this recession progressed the moraine-material, no longer washed out and distributed over the sea-bottom as gravel, was left behind in a continuous form as unstratified Glacial clay. Some of it, perhaps the greater portion, remained as extruded ; but part of it was carried at the bottom of bergs as they broke off, and dropped as before described, partly over the previously spread-out gravel and partly over or onto previously extruded moraine-material which had been left undisturbed. The presence of the thin sand- bed in the midst of this unstratified clay with rolled chalk at the base of Dimlington Cliff, in Holderness, which contains Nucula Cob- boldice and other Mollusca in the condition in which they lived, is thus explicable ; for the Mollusca having established themselves on the surface of the submarine moraine which the receding ice had left, were afterwards covered up and destroyed by the descent from the bottom of a berg of a mass of the subsequently extruded mo- raine-material which such berg had carried away. Thus the forma- tion of Glacial clay went on principally in the immediate contiguity of the ice, where it arose from the extruded material being left by the ice in its recession, and subordinately at a little further distance from it, where it arose from the same material being dropped on the sea-bottom over previously accumulated formations. So long as the land-ice rested upon Lincolnshire and South York- shire this moraine-material w T as principally made up of the debris of the Chalk and of the softer beds of Jurassic age ; but as it shrank back into Yorkshire, this debris, gradually lessening, eventually ceased, its place being taken by the debris of formations lying north of the Yorkshire Chalk Wold with which the clay without chalk debris that both overlies the clay with this debris and gradually takes its place northwards is filled. A glance at our map in the ' Introduction to the Crag Mollusca ' Supplement, will show that in South-east Suffolk and in Northern Norfolk the Middle Glacial coming out from beneath the upper, extends far beyond it, a solitary patch or two of the latter occurring here and there over the former, indicating some excep- tionally distant drift of the moraine-material ; and a similar feature is presented by North-east Essex, and, to some extent, by Hertford- shire also. In this we get a precisely parallel feature to that pre- sented in the overspread of the Upper Glacial clay with rolled chalk debris in Holderness, by the later clay without that debris ; and in this respect and in the horizontal succession of the older deposit by the newer in a northerly direction, the relation of the Middle Glacial to the Upper in East Anglia seems identical with it. The parallel may be even pursued further ; for just as the clay destitute of chalk debris is but thinly spread over the clay containing it, and assumes far greater thickness as it takes its place horizontally, so is the thick- ness of the Upper Glacial where it rests on the Middle, but small in