Page:Popular Science Monthly Volume 34.djvu/40

30 terminal moraine to Philadelphia. They are most conspicuous in the great gravel terrace just south of Belvidere, gradually diminish in volume and height and even merge into the modern alluvium by which they are in part overlaid between Easton and Trenton, become conspicuous again at Trenton (where they cover an area of fully fifty square miles, and are exposed in every natural and artificial excavation below their maximum altitude in and near the city), and finally disappear near Bristol, though the cobbles are largely dredged from the channel to and beyond Philadelphia. They are in part overlaid by modern alluvium, into which they appear to merge midway between the moraine and Trenton; and they repose unconformably upon the greatly eroded surface of the Columbia formation—the aqueo-glacial deposits of the earlier cold epoch of the Quaternary—notably at Trenton, where they fill a basin lined with Columbia brick-clays and gravels.

By structure, composition, and topographic relations the deposits tell the story, as by their geologic relations they fix the date, of their origin. At Trenton the deposits consist of stratified gravels more heterogeneous than, but otherwise undistinguishable from, those of the terraces into which the terminal moraine merges, interspersed with bowlders up to one hundred cubic feet in volume, the whole imbedded in a matrix of sand and loam. The entire mass is unquestionably water-laid; its continuous bedding is indicative of wave-action, and thus of shallow waters; and the bowlders scattered throughout it are evidently ice-borne. Its structure is shown in Fig. 3, reproduced from a photograph taken in the extensive gravel-pit half a mile east of the depot at Trenton. The relations of these gravels to the subjacent Columbia formation are shown in Fig. 4, also reproduced from a photograph taken at Chambersburg—the coarse, stratified gravels.