DEPTH OF THE FURROWS 129 



From these facts it appears that if the drift were wholl}^ removed and 

 the rock surface bared the topography would comprise a series of ridges 

 and troughs running parallel to the trend of the northeasterl}'^ streams. 

 The details of rock configuration conform to the direction of ice motion, 

 and are evidently products of glacial erosion. 



The depth of the furrows is not definitely known, but some of them 

 exceed 40 feet. Measurement was largely dependent on the facilities 

 afforded b}' surface wells, and these do not completely traverse the drift 

 in the deeper furrows. The significant fact is that throughout a consider- 

 able area the jjreglacial topography, presumabl}^ including a system' of 

 shallow valle5'S descending northward, was obliterated, and a radically 

 different system of valle3^s was wrought. This could hardly have been 

 accomplished without a general reduction of the surface to the extent of 

 40 or 50 feet, and the amount may have been considerably greater. The 

 amount was surely greater in the vicinity of the Niagara river, where 

 erosion, as already mentioned, not onl}^ carried the outcrop of the chief 

 sandstone ledge back to the line of the Niagara escarpment, but removed 

 so much of the underlying shale that the sandstone outcrop now con- 

 tours the face of the escarpment for some distance above its base. 



The failure of the glacial furrows to control the courses of Eighteen- 

 mile creek and Niagara river is due to local conditions. At each of these 

 points the lowering of glacial lakes to the plane of lake Iroquois was ac- 

 companied by a stupendous torrent pouring over the Niagara escarp- 

 ment,* and the detritus thus borne northward helped to silt up the gla- 

 cial furrows of the plain. The subsequent lake action completed the 

 w^ork of gradation, so that when Iroquois was drained awa}^ these two 

 streams found lines of continuous descent before them and chose direct 

 courses to lake Ontario. 



Summary 



The ice left the broad plateau of firm Niagara limestone with little 

 modification. Tbe cliff at its northern edge was somewhat worn, but its 

 position not materially changed. The resistant ledges north of the 

 Niagara limestone were worn back so far as to lose the contours typical 

 of subaerial erosion. The topography of the Medina shale was recon- 

 structed, its system of stream valleys and interstream ridges being re- 

 placed by glacial fluting on a large scale. While the Niagara escarp- 

 ment antedates the period of glacial sculpture, ice erosion rendered it 



* Bull. Geol. Soe. Am., vol. 8, p. 286. 

 XIX— Bull. Geol. Soc. Am., Vol. 10, 1898 



