Page:Popular Science Monthly Volume 34.djvu/41

 bowlders, etc., representing the later deposit, and the homogeneous loam passing downward into coarse gravels representing the older formation. The thickness of the deposit ranges from a trifling veneer to forty feet or more; and its surface, where it has escaped erosion, forms a plain inclined gently southward from an altitude of about forty-five feet in north Trenton to tide-level midway between Bristol and Philadelphia; this inclination of the deposit being the measure of the northward tilting of the land during the later ice epoch.

Within the Trenton gravels two types of implement are found—viz., "turtle-backs" and the rude "leaf-shaped" implements regarded by Abbott as of Esquimau pattern. Both types are chipped from a peculiar argillite which is found in the deposit only as (presumptively) finished implements or as large bowlders. The implements, which occur in such numbers that over twenty-five thousand have been collected by Abbott, are seldom water-worn though frequently weathered, while the bowlders are somewhat worn by water and similarly weathered. It is significant that the "turtle-back" type is found throughout the deposit from top to bottom, but most abundantly in the lower half and in progressively diminishing abundance from bottom to top of the upper half, while the "leaf-shaped" type is found only in the upper half and in progressively increasing abundance upward; it is also noteworthy that both types of implement are occasionally found over contiguous surfaces of the Columbia formation (which were above water-level when the Trenton gravels were deposited), commonly associated with chipped implements of higher type; and it is equally noteworthy that the implements of higher type occur over the surface of the Trenton gravels but never within them, while the ruder implements found within the gravels do not occur upon the gravel surface.