Page:Quarterly Journal of the Geological Society of London, vol. 26.djvu/206

106 should, however, mention that the formation of this clay must, I think, have commenced when much of the country to the south of the sea-limit, represented in the sketch map (p. 102), was uncovered by sea, because the junction of the chalky clay with the Middle Glacial sands indicates an uninterrupted succession of deposit, the change, abrupt as it is, being only in the material deposited, viz. from sand to Boulder-clay. Coupling this with the indications everywhere pointing to the circumstance that, during the Middle Glacial period, the sea does not appear to have stood more than from 400 to 450 feet above the present sea-level, we must suppose the area of sea represented in the map, to the south of the ice-sheet, to have been considerably less at the commencement of the chalky-clay deposit than at the stage selected for illustration. Moreover I hope, when describing the Middle Glacial structure, to be able to show that land much below this elevation was, during the Middle Glacial deposit, occupied with ice which blocked out the sea of that formation, precisely as I have represented it (in the triple section) as doing in Yorkshire during the chalky-clay deposit. It seems to me also to follow, from the facts touched upon, that it was during this interval of pause in the subsidence, when intense cold prevailed, that the truly Arctic fauna of Bridlington, which so contrasts with that of the Middle and Lower Glacial formations, gradually became established, and that thus, the Arctic forms having become denizens of this part of England by the close of this interval, we find it in full force in the Bridlington bed, at an horizon in the deposit which indicates a very early stage in this renewed subsidence — that is to say, in the lowest or chalky part of the purple clay. On the other hand, so far as the mollusca yet obtained afford an indication, there is ground for supposing that during the period in which the whole of that thick body of deposits forming the Cromer cliff was accumulated, and during that of the Middle Glacial sands which succeed and overlie it, no arctic mollusk, beyond such as occur in the Crag, except Tellina balthica, established itself in these parts. The introduction of Tellina balthica constitutes a very clear palaeontological horizon, marking the commencement of the Glacial formation, which (as I hope on a future occasion to show) exactly coincides with a physical break and unconformity with the Chillesford beds and Crag that takes place at the horizon where this shell first makes its appearace; and that