Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/39

Rh extensive area of land with a gentle slope, such as Greenland, produces similar effects by slowly moving down to the sea shore; and it is probable, too, that the great icebergs, some of which are 4 miles in circumference, and 1800 feet high, would, when grounding and moving along the sea bottom, also polish and striate rock surfaces. The love of simplicity has, perhaps, led theorists to attribute too much to one cause. The question presented for solution by the phenomena of the boulder clay is complicated; for, as conditions in the problem, there are not only glaciated surfaces and glaciated boulders mixed pell mell in the lower clay, but there are similar boulders in gravel and sand beds, and also stratified sands and clays, and water-worn gravels; and in addition, the occurrence of marine shells in even the lower clay. A comprehensive theory must, therefore, allow for the play of various agencies—the movement of ice down mountains, and over extended areas of land to the sea shore, the stranding of icebergs, the transport of gravel and sand by marine currents, and even the quiet deposit of mud in stiller waters, during the long period comprehended in the boulder clay formation.

2. Post Plistocene.—Since the boulder clay era, there has been little change in the general contour of the district: some valleys have been deepened, some shallow lakes and marshes drained; and along the coast can be found traces of slight oscillations of level. Gravel, very much rolled and rounded with beds of sand, chiefly seen in. river valleys, accompanied with terraces on the river banks, belong to a later period than the boulder clay. More recent, still, are accumulations of peat, which is formed at all levels from the sea coast to the summit of our highest hills, under the conditions of moisture and cold. A depression of the land is indicated by peat and overthrown trees (sub-marine forests) seen in some parts of the coast, below the line of the lowest tides: a marked case of this character is near to Howick. Frequently below the peat is a deposit of marl, which, in some instances, is filled with fresh-water shells, all of recent species. But besides these there have been found, either in the peat or in the marl, Bos primigenius, Bos longifrons,