CLINTON AND MEDINA LEDGES 125 



of the plain, and their configuration is in all respects characteristic of 

 glacial erosion. They range in depth from 10 to 30 feet, are usually 

 several hundred feet broad, and the longest probably extends more than 

 a half mile into the plain. The diagram in figure 4 shows the general 

 character of the rock contours, but without representing any individual 

 contour at equal height above the sea. The equivalent topographic out- 

 line is less sinuous, because the rock furrows are largely filled by drift. 



UWISTON 



I Miles 



Figure 4. — Contour on the Niagara Limestone at the Niagara Escarpinent. 

 The escarpment faces north. Arrows show observed directions of glacial strife. 



The configuration of the cliff seems to show that in the regions where 

 the trend is southwest all minor salients have been pared away by the 

 ice, and that where the trend is southeast minor irregularities of the face 

 have been exaggerated and small reentrants drawn into furrows ; but the 

 principal salients and reentrants of the topography are preserved, and 

 ice modification is limited to minor details of form. 



Considering this evidence of cliff sculpture, in connection with the 

 removal of the limestone in the Middleport region and the non-glacial 

 shapes of the rock ridges of the upper plain, it would appear that the 

 ice-sheet concentrated its work, so far as the Niagara limestone is con- 

 cerned, on the crest of the escarpment, and that even there its results were 

 of secondary rather than i)rimar3' importance. Probably the limestone 

 at its escarpment lost on the average only 10 to 20 feet of thickness, and 

 from the broad belt of outcrop the general loss may have been as small 

 as 5 feet. 



Sculpture of the Clinton and Medina Ledges 



Beneath the Niagara limestone are 80 feet of shale belonging to the 

 Niagara formation ; then 25 feet of limestone (Clinton), and then a great 

 bod}^ of shales (Medina and Hudson River), containing a few sandstone 

 lenses near the top. The Clinton limestone and the sandstone ledges 

 are strong beds, but the remainder of the section consists of weak shale, 

 opposing little resistance to erosion. The Clinton limestone is continu- 

 ous and uniform, the sandstones discontinuous. The sandstones lie at 

 various depths below the limestone, ranging from 40 to 100 feet. The 

 heaviest sandstone is 20 feet thick at Lewiston, diminishes eastward in 

 a few miles to 4 feet, then thickens to 14 feet, and finally disappears near 



