GEOLOGY OF THE REGION 337 



complex. One of these^ exposing an impure limestone, is crossed by the 

 Susqnehanna at Shenks Ferry, but is apparently without influence on 

 the profile or plan of the river valley. 



The rocks composing the overthrust block are micaceous schists and 

 gneisses and metamorphosed eruptives. With minor exceptions, they 

 seem to offer the same degTee of resistance to the erosion of the river. 

 The outstanding salients of the gorge do not correspond with noticeable 

 differences in the cliaracter of the rock, but are largely determined by 

 the eu trance of side streams, wldcli away from the gorge flow back and 

 forth over geological boundaries little influenced l)y tlie differences in 

 tlie resistant qualities of the underlying formations. 



The schistosity and possible bedding of the rocks strike across the gorge 

 and dip at steep isoclinal angles for miles at a stretch. The geologists 

 of the Second Pennsylvania Survey noted an anticlinal fold crossing the 

 river above McCalls Ferry in the vicinity of T'orquan Creek, and later 

 studies on both sides of the river have adduced evidence of a series of 

 open folds in accord with the Appalachian structure to the north and 

 west. 



The surface of the thrust-block and the underlying formation, where 

 exposed, show the Somerville, Harrisburg, and older plains. These, how- 

 ever, are not well shown on the sides of the gorge, though readily recog- 

 nized along the side valleys and in the upland area on either side of the 

 river (the apparent single plain shown in plate 18, figure 1, is in reality 

 composed of several plains at different elevations, as may be seen from a 

 study of the topographic maps). At a few points, as near Washington- 

 boro, Pennsylvania, and Wildcat Point, Maryland, there are cappings of 

 '^'high-leveP' gravels. Below the Somerville plain are evidences of one 

 or more still later incipient baselevelings in the level rock floor of the 

 river itself and the low-lying islands and contiguous rock flats. These 

 latest beginnings of baselevels are provisionally called tjie Susquehanna 

 baselevel for convenience, although it is clearly recognized that they in- 

 clude little more than local wicTenings of tlie floor of the gorge. Tt is in 

 the latest of these plains that the river has cut the "deeps" which form 

 the subject of this paper. 



Detailed Description of the "Deeps" 



^ LOCATION OF THE "DEEPS" 



The well defined "deeps^^ disclosed by the careful hydrographic survey 

 of the river bottom condncted by the McC'alls FeriT Power Company are 

 locafed respectively 1% miles above Safe Harbor, above the dam at Mc- 



