Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/264

192 forming an escarpment roughly parallel to that of the Chalk. The northern and eastern portions of the district are in part occupied by deposits of later Tertiary age— he Crags, the Chillesford beds, and the Forest-beds.

The general altitude of the line of escarpment of the Upper Chalk may be taken as from 300 to 500 feet, that of the London Clay from 200 to 300 feet above the sea.

Pliocene.—Along the coast of Norfolk, Suffolk, and part of Essex, as well as for some distance inland, occur the "Crag" deposits. Of these sands, the older or "Coralline Crag" contains the remains of a deep-sea fauna; in the " Red Crag," immediately succeeding, littoral forms prevail; while the newer or "Norwich Crag" also contains littoral forms with land and freshwater shells in addition. A gradual rising of the coast during the Crag period is thus indicated; and this continued until the land stood slightly higher than at present, admitting the growth of the Cromer "Forest-bed."

These Crag-beds perhaps scarcely belong to the drift-deposits that present the evidence on which the following arguments are founded, although I believe that there is a gradual passage up from them to those of recent date. This gradual passage will probably be found to exist stratigraphically, as well as in regard to their enclosed fossil remains and to the climate that prevailed during the different periods of their deposition.

Lower Glacial.—After the formation of the Cromer Forest-bed the land again sank beneath the water, gradually and to a depth of certainly not less than 400 or 500 feet : during the progress of the submergence the advancing shore-line gave rise to the "pebbly sands" which, according to Messrs. Wood and Harmer, "form the base of the whole glacial series, indicate shore-conditions and the first setting-in of the great glacial subsidence". Arctic conditions of climate began to prevail; and here and there patches of clay were dropped by icebergs, heralds of those to follow in such vast numbers during the Upper Glacial epoch. The contorted drift, which occupies a large area in the north-east portion of the district, was deposited during the early part of this submergence of the land; its mode of formation I consider to have been much the same as that of the Upper Boulder-clay presently to be considered. The contortions sometimes exhibited by this deposit are probably due to agencies acting on the clay at a subsequent period.

Middle Glacial.—The Lower Glacial beds are overlain and considerably overlapped by a series of gravels and sands called the "Middle Glacial." These beds occupy, or have occupied, almost all the area covered by the Lower Glacial, and extend far beyond it in a southerly direction. It will be seen on reference to the map (fig. 1,