PENNSYLVANIA AND DEPOSITS THEREIN. 57 



a calm ocean, and bends up stream to show the above movement. It 

 is so large that it has forced the river to make a loop i mile to the 

 north of its straight course in order to pass around it, and a remnant 

 exists near its former end as an island i mile long. Figure 3 shows 

 the rough assortment of fresh Oneida sandstone and limestone in 

 the dark and fully decayed preglacial surficial mantle of Mosquito 

 Valley. Smaller ridges are found to the east, and opposite similar 

 gaps in Bald Eagle Mountain, composed of similar materials from 

 smaller hopper-shaped valleys. Excepting the most eastern, they 

 bend up stream with the reversed current : the other bends down 

 stream with the discharge through the marginal canyon. All have 

 forced the Susquehanna to make loops to the north to pass around 

 their ends. 



The flood plain of this stream is here composed of rocks between 

 the Lewistown limestone and the Pennsylvanian. It is evident that 

 these ridges of entirely underlying measures are not part of a com- 

 plete valley filling subsequently excavated ; but are like the long 

 ridges in the Lehigh Valley, dropped in deep ponding as soon as the 

 velocity of the carrying torrent was sufficiently checked. 



PART TWO. 

 Allegheny Pondings. 



Introduction. 



We note from what has been described above that ponding may 

 be caused by stream reversal by glacial agencies, and by ice-dams. 

 Permanent stream reversal is brought about by the trenching of the 

 col or saddle in the watershed of the reversed stream to a depth suf- 

 ficient to ensure the permanent discharge of the accumulated water 

 after the wasting of the glacier which caused the original ponding. 

 Secondary pondings, thereafter, are formed by bergs, as in the 

 "Narrows" of the Delaware, the North Branch of the Susquehanna, 

 and the Juniata. 



Figure 4 follows Leverett's Fig. i^ in the arrangement of the 

 three river systems that now make up the Allegheny River. Instead 

 of " Old Upper," " Old Middle " and " Old Lower " Allegheny, these 



