the Middle Atlantic Slope. 375 



as on the Potomac is a sub-estuarine delta laid down during a 

 period of cold and submergence ; and the differences prove that 

 the submergence and the refrigeration were both the greater 

 on the former river. 



Above the fall-line the Susquehanna, unlike the Potomac, 

 is flanked by deposits corresponding to the sub-estuarine delta. 

 Between Columbia and its mouth, it is true, the river flows rap- 

 idly through a steep-sided gorge of considerable depth, the tribu- 

 taries have high declivity, and superficial deposits are not pre- 

 served ; but above Columbia the valley widens, its slope di- 

 minishes, and remnants of slack water deposits appear. Four 

 miles above Harrisburg the river breaks through Kittatinny 

 mountain in a widely-known water-gap, and embouches upon a 

 slightly undulating terraced plain 100 to 200 feet above its 

 level ; and the prevailing superficial deposit over this plain is 

 loam or brick clay passing down into a gravel or bowlder bed. 

 A representative section of the prominent terrace half a mile 

 northeast of Harrisburg is as follows : 



1. Fine loam, massive above and horizontally laminated below, 



with a few disseminated pebbles and layers of sand toward 

 the base, largely used as a brick clay ... 7 feet. 



2. Irregularly stratified gravel, comprising pebbles (commonly 



rounded) from 3 inches downward, imbedded in a matrix 

 of coarse brown sand, the shale deeply ferruginated and 

 sometimes cemented . . . ..... 4 feet. 



3. Stratified coarse brown sand abounding in pebbles and 



bowlders 3 feet. 



The three members are here sharply demarked, but elsewhere 

 intergraduate. 



Save that these deposits at Harrisburg are somewhat thinner, 

 that the bowlders are smaller, and that they are without Pied- 

 mont crystallines, they are scarcely distinguishable from 

 those about the head of Chesapeake bay. There is the same 

 brick-reel color, the same degree of ferrugination, the same bi- 

 partition and the same structure in each member, the same 

 black ferruginous cement uniting and staining the pebbles, the 

 same intergraduation of the members, and indeed so close 

 similarity in all essential respects that either deposit might be 

 accepted as the type of the other. And the deposits at Harris- 

 burg are representative of those of a considerable area: the 

 tract mantled with brick clay and gravel on the north side of 

 the river below the Kittatinny water gap is 10 miles long 

 and 5 miles wide, and the area of the deposits on the south side 

 of the river is nearly as great. Above the water gap the de- 

 posits are still more largely developed ; mile after mile the 

 Susquehanna is flanked by gently sloping plains descending 



