380 W. J. McGee— Three Formations of 



gravels. To the northward the high level loam and bowlders 

 pass beneath the terminal moraine, and the overwash gravels 

 graduate into the hillocky debris of the drift-lined valley as on 

 the Susquehanna ; and as on that river too the overwash gravels 

 rapidly diminish in size and abundance down stream, the ter- 

 races meantime merging and decreasing in height, until both 

 practically disappear above water level 10 miles below Bel- 

 videre. Local accumulations of the overwash gravels occur, 

 however, at various lower points on the river, the last and 

 most conspicuous being at Trenton, where the later-glacial Dela- 

 ware river opened into a broad estuary in which the vernal 

 ice-floes dropped their debris gathered at the ice front. 



On the Delaware, as on the Susquehanna, the two series of 

 superficial deposits — the moraine with its derivatives and the 

 terraced brick clay with its gravels — are perfectly distinct and 

 widely diverse in age; and here, too, the deposition of the 

 loam and high level gravels must have been accompanied by 

 transformation of the Delaware river from a rapid unnaviga- 

 ble stream abounding in cascades and rapids, to a tidal estuary 

 miles in width within which fine silt and clay were dropped, 

 and upon which bowlder-bearing ice-blocks floated — the land- 

 submergence reaching fully 400 feet in the latitude of the 

 terminal moraine. 



The Deposits on other Rivers. — Every considerable stream of 

 the Middle Atlantic slope has at the fall-line a conspicuous 

 deposit analogous to those of the Potomac, Susquehanna, and 

 Delaware ; and while the deposits vary in volume with the 

 streams, the structure, the composition, the geographic and 

 hypsographic relations, etc., remain constant or change slowly 

 with latitude. The Schuylkill and Brandywine deposits merge 

 into those of the Delaware, but in their up-stream extension 

 are distinguishable therefrom by the abundance of local and 

 the sparseness of northern materials ; the deposits of Elk and 

 Northeast rivers are distinguishable from those of the Delaware 

 on the one hand and of the Susquehanna on the other by the 

 preponderance of local materials, and at low levels by their 

 independent terrace systems ; the Patapsco deposits merge into 

 those of the Susquehanna at Baltimore, but local pebbles in the 

 lower member and the preponderance of local residuary debris 

 in the upper give individuality to the Patapsco. delta, which in 

 structure, composition, and general aspect, is undistinguishable 

 from that of the Potomac river at Washington ; the two 

 branches of the Patuxent have beautifully terraced deltas ex- 

 hibiting characteristic bipartition and all of the diagnostic 

 features of the fluvial phase of the Columbia formation ; and 

 the Anacostia has an independent but homologous system of 

 deposits and terraces made up predominantly of materials de- 

 rived from the marginal portion of the Piedmont area. 



