Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/153

Rh LATER TERTIARY GEOLOGY OF EAST ANGLIA. 115 tudes of 700 feet in Gloucestershire, showing that the submergence over that part of England had at this time reached at least that extent. This branch, as it seems to us, and as we shall presently explain, reached as far south as the borders of the counties of Buckingham. Bedford, Hertford, Essex, Suffolk, and Norfolk during the formation of the Middle Glacial, the sand and gravel of which we regard as produced by powerful currents washing out and searching the mo- raine of this branch, and distributing the insoluble residuum over the sea-bottom. By the time when this branch of the ice-sheet reached the counties just named, not only had the East-Anglian valleys which the sea had first filled and converted into fiords become engulfed, but the whole country beyond the limit of the land-ice had become sub- merged to the extent of near 400 feet. South of the limit which we shall presently define, it seems to us the land ice-sheet never extended ; and the formation of the Upper Glacial marks its gradual recession and disappearance. This formation was represented in a paper by one of us* as formed by the extrusion by the ice-sheet of its moraine and the distribution of this at the bottom of bergs. Such we still believe to have been its origin. In some instances, as where the Middle Glacial passes up, as it often does, by gradual change into the Upper, this seems to have been due to the fall of the moraine- material from the bottom of the bergs in small quantities ; but more generally, as where the Upper Glacial rests sharply and irregularly on the Middle, the dropping seems to have occurred in large sheets or masses. Where the latter has occurred, it has frequently hap- pened that the clayey mass, having fallen on sand saturated with the sea-water, and therefore semifluid and yielding, has sunk into it, though without contorting it, and so given rise to the appearance which was at first supposed by us to indicate a slight fault. Where the dropping of the mass occurred on less-fluid though still yielding material, such as the brick-earths of the Contorted Drift, it has sunk into or penetrated it to some extent, but also without contorting itf ; and herein is presented a remarkable con- trast to the introduction of the marl masses into the Contorted Drift, which, being invariably accompanied by contortions, was evidently due to the grounding of bergs during the accumulation of the deposit itself. It is obvious that the hypothesis of the Middle Glacial having been deposited while the eastern side of England was submerged to the extent of about 400 feet, offers no explanation of the absence of that formation over a large district of England which lies north of it and forms low ground, unless we are prepared to suppose that a great change in the relative levels of this part of our island t A striking section of this is to be seen in a great excavation a quarter of a mile east of Yarrow House, and nine furlongs south of the bridge over the Wensum at Guist, a very little out of the line of Section VIII. Q. J. G. S. No. 129. i
 * Quart. Journ. Geol. Soc. vol. xxvi. p. 90.