450 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



by a series of violent rapids, the water tumbling over a rough bottom 

 composed of limestone blocks fallen from the walls. Beyond the 

 Niagara escarpment the river traverses a low plain with deep and quiet 



current. 



In the walls of the canyon the strata are finely displayed. At thetop 

 the Niagara limestone has a thickness of about 80 feet (24 m.) near the 

 cataract, and this gradually diminishes to the edge of the plateau, the 

 difference being due to the general degradation of the surface. The 

 fall thickness of the limestone previous to erosion was about 1 10 feet 

 (42 m.). Beneath the limestone is the gray Niagara shale, about SO 

 feet (24 m.) in thickness; then come the Clinton beds, gray limestones 

 and shales, with a sandstone at base, and a total thickness of 36 feet 

 (10 m.), and finally the Medina shales and sandstones, here consisting 

 Chiefly Of red arenaceous shale rarely interrupted by ledges of sand 

 stone. At the foot of the cataract, the Clinton is near the water's edge; 

 northward it rises at the rate of 25 feet (7 m.) to the mile, and the 

 river falls at a much more rapid rate, so that a basal exposure of the 

 red Medina increases rapidly from the cataract to the escarpment All 

 these beds, except the Niagara limestone, are in places more or less 

 obscured by talus, but the complete section can be seen on the Ameri- 

 can side, just below the lower suspension bridge. 



The basins occupied by Lake Brie and Lake Ontario had a different 

 system of drainage previous to Pleistocene time, and were remodeled by 

 the work of the ice sheet, which modified the geography of the Great 

 Lake region in important ways. Some regions of soft strata Buffered 

 notable erosion, and the old drainage lines were in many cases com- 

 pletely obliterated by deposits of the glacial drift. When the ice 

 melted the waters were compelled to find new ways, and the drainage 

 of the glaciated region was imperfect or immature, in that it included 

 an immense number of lakes, large and small. Lake Brie and Lake 

 Ontario came into existence at that time, and so did the Niagara river. 

 The erosion of the Niagara gorge from the escarpment back to the 

 cataract is therefore a post-glacial work, and as a measure of post 

 glacial time it has attracted great attention. The length of the gorge 

 eroded, about six miles, is readily measured. The present rate of ero- 

 sion by the cataract is susceptible of measurement, and observation 

 lias already given it a value with valid claims for consideration. From 

 a survey made in 1842, and subsequent surveys made in L875, 1886, 

 and 1890, it appears that the central portion of the main cataract, the 

 Horseshoe fall, is receding 4 or 5 feet per annum, and that the Ameri- 

 can fall, which carries much less water, is receding much less rapidly. 

 For those who are willing to postulate a uniform rate of recession 

 through the whole extent of the gorge, it is easy to estimate the age of 

 the river from these data, but there are important reasons for question- 

 ing the validity of the postulate. 



