172 G. F. Wright — Continuity of the Glacial Period. 



considerably in width, depending upon the ability of the rocks 

 to resist or facilitate erosion ; while sometimes the shelf is 

 wholly wanting for a considerable distance where the river 

 passes through areas of more uniform and compact strata. 

 But almost everywhere down to the junction of the Big 

 Beaver with the Ohio this 200-foot rock shelf is covered with 

 glacial gravel to a depth of from twenty to sixty feet and 

 sometimes for a width of a half mile. The extent of the de- 

 posits of this high-level terrace has not heretofore been duly 

 understood or appreciated. The portions heretofore described 

 at Bellevue on the Ohio, just below Pittsburgh, and that at 

 Parker's Landing, on the Allegheny, can be duplicated at brief 

 intervals throughout the entire length of the valley from the 

 mouth of the Conewango to the mouth of the Big Beaver. 

 The deposits are found almost continuously upon the east side 

 of the Ohio River, from the mouth of the Big Beaver to 

 Pittsburgh, while in Allegheny City remnants of the gravel 

 remain upon the summit of Monument Hill, which rises in the 

 center of the city to the level of the rock terrace, but is sepa- 

 rated from it by a quarter of a mile or more of a partially 

 buried old channel of the river. Under guidance of Professor 

 Jillson, who will soon publish a detailed paper upon the sub- 

 ject, I traced this high gravel terrace for some twenty miles 

 up the Allegheny River. It is developed in specially good 

 degree on both sides of the river above Allegheny City and 

 Pittsburgh, and farther up near Verona, upon the south side 

 of the river, and on the north side opposite Parnassus, and 

 again from Tarentum for several miles upon the same side 

 nearly to Freeport. In all these places the accumulation of 

 gravel upon the rock shelf is from twenty to sixty feet in 

 depth, and frequently is as much as a half a mile wide. The 

 material is much of it fine and often well stratified, but con- 

 tains numerous water-worn Canadian pebbles and occasionally, 

 enclosed in the fine material, angular fragments of granite, 

 gneiss, or sandstone two or three feet in diameter, indicating 

 ice-laden currents. At East Liberty, in the upper part of 

 Pittsburgh, the deposit from the Allegheny River crowds over 

 upon a limited amount of river pebbles which had been dis- 

 tributed along the deserted bed of the Monongahela which is 

 now followed by the Pennsylvania Railroad in getting out of 

 the city, and which, is about the same height as the rock shelf 

 referred to. Farther up the Allegheny River, in company 

 with Mr. D. C. Baldwin, I have examined similar deposits 

 along this rock shelf at Ford City, Kitanning, Red Bank, East 

 Brady, Parker's Landing, Kenerdell (formerly Scrub Grass), 

 and many other places. Of these I will now speak further 

 only concerning Kenerdell and Parker's Landing. 



