POT-HOLES IX PEESEXT EIVEE BED 301 



sive. As similar proof may be available at other places where the slope 

 of the old bed is not definite, or may even be negative, it seems desirable 

 to give more fully the facts showing the correctness of the opinion here- 

 tofore held in reference to them. 



At and above Rock point, for about a mile, the aid fluvial plain is in the 

 Homewood sandstone, which here is a hard, massive stratum. It is well 

 glaciated, being striated over quite an area. These markings, evidently 

 due to the last advance of the ice in this region (the Wisconsin ^) belong 

 to a time much later than the erosion of the old fluvial plain, and show 

 the great resisting powers of this stratum. 



About 10 miles south of Rock point the same stratum now forms the 

 bed rock of the present stream, which is marked by pot-holes from Beaver 

 Falls dam to the edge of the stratum, where it is cut by the now buried 

 channel of the Beaver, below the New Brighton-Fallston clam. The 

 stream bed is especially eroded below the last-named dam, presenting 

 such a number of pot-holes that many of them have been cut and en- 

 larged until they have broken into adjoining ones. The locality is typical 

 of this phase of stream erosion. It is especially advantageous to study 

 the effects of erosion here in order that the *' live " pot-holes may be com- 

 pared with the " fossil " ones found in the same stratum near Rock point. 

 It is also to be noted that the local character of the Homewood sand- 

 stone is almost absolutely identical with the same stratum at Rock point. 



In the accompanying illustrations, plate 32 is a general view of the 

 Beaver below the Fallston dam. looking diagonally up the stream. An 

 examination of this will show the abundance of the phenomena to be 

 found here, so much so that it is difficult at some places to get isolated 

 examples for illustration. Attention is directed to the fact that the steep 

 side of the pot-hole is in all cases the upstream side of the hole, while 

 the downstream side has been rounded off and eroded by the impact of 

 the water. Attention is also directed to the general slope of the bed of 

 the stream, being almost 10 feet in the short distance shown in this figure. 

 This is a somewhat greater fall than usual, due to the nearness of the 

 buried channel, but a rapid flow of water is a requisite in the formation 

 of pot-hole structure. Further reference to this will be made in apply- 

 ing the fact here noted to the locality at Rock point. 



Plate 33 shows somewhat in detail three pot-holes which have not yet 

 succeeded in entirely cutting away their dividing walls. This view, 

 taken almost against the stream's flow, shows on the upper side of each 

 pot-hole the steep, abrupt edge, indeed undercut, and on the downstream 

 side that the rim has been rounded off" and eroded. In each of the pot- 

 holes the bottom is filled with water, as shown in the figure. 



♦Leverett : U. S. Geol. Sur. Mon. xii. 



