TOPOGRAPHY. 71 



regularity of the contours in the portions trending northeast to southwest 

 is here shown to be sti'ikingly in contrast with the portions that trend 

 northwest to southeast. The effect of glacial erosion on the brow of the 

 escarpment appeal's therefore to have been insufficient to remove the sahents 

 and reentrants of the preglacial topography. It is estimated by Gilbeit 

 that the hmestone at its escarpment lost on the average only 10 to 20 feet 

 of thickness, while from the broad belt of outcrop the general loss may 

 have been as small as 5 feet. The minor ridges of the limestone surface 

 do not conform in trend to the direction of the ice motion, as do the ridges 

 of the Medina shale, and the amount of erosion in the limestone is thought 

 to be scarcely one-tenth as great as in the shale. The discriminating- 

 studies begun by Gilbert promise to throw much light upon the question of 

 the share of work borne by ice in the production of the topographic features 

 of the glaciated districts, including that of the origin of the basins of the 

 great Laurentian lakes. 



To this escarpment is due the great cataract of Niagara and the lower 

 or Rochester Falls of the Genesee River, as well as several falls in small 

 streams that pass over it. At the Genesee River and on Oak Orchard 

 Creek cascades occur at the points where these streams cross the Medina 

 sandstone and Clinton limestone as well as the Lockport limestone. The 

 Niagara cataract has only the one fall, which extends from the top of the 

 Lockport limestone down below the Clinton, with a submerg-ed gorge 

 extending down into the Medina shale. The writer lias nothing to add to 

 Hall's interpretation of the development of the three falls on the Genesee 

 at Rochester.^ The falls on the smaller streams may be passed with this 

 simple mention of their occun-ence, since the writer has made no special 

 study of them. 



PLAIN SOUTH OF NIAGARA ESCARPMENT. 



Between the Niagara and Corniferous escarpments there is a plain 10 

 to 15 miles in width which, as indicated by the topographic map (PI. Ill), 

 descends for a few miles south from the Niagara escarpment and then rises 

 gradually toward the base of the Corniferous escarpment. It thus forms a 

 shallow trough affording a natural avenue for drainage along its axis. The 

 western portion is utilized by the lower course of Tonawanda Creek, a 

 tributary of the Niagara River; the middle portion by the upper course of 



1 New York Geol. Survey, Fourth Gaol. District, 1843, pp. 381-382. 



