28 



average top width is about 750 feet (228 m.) ; the average 

 width at the water line is about 250 feet (106 m.). 

 Knowing this and the volume of the river and the rate of 

 flow in the rapids, Mr. Gilbert estimated the depth to be 

 35 or 40 feet (10-7 to 12-2 m.). Spencer's soundings 

 from the upper railway bridge show a midstream depth 

 of 86 feet (26-2 m.) shallowing towards the sides, but the 

 water there is only beginning its descent and has not 

 acquired its full velocity. In the lake history, this section 

 is the correlative of the Nipissing great lakes, and was 

 made, therefore, when the falls carried only the discharge 

 of Lake Erie. Spencer interprets this section in an entirely 

 different way, making it 185 feet (56-4 m.) deep and choked 

 up with fallen blocks to a depth of 100 feet (30-4 m.) or 

 more. The. views of Pohlman, Grabau and others are 

 given below in the discussion of the Whirlpool and the 

 St. David buried gorge. 



The Upper Great Gorge. 



This section, reaching from the head of the narrows 

 just above the railway bridges to the Horseshoe falls, is 

 about two and one-fourth miles (3-8 km.) long. Opposite 

 the American fall it is 1,600 feet (487 m.) wide at the 

 top and is wider than the average from there to the south 

 side of Goat island. Its average width north of the Ameri- 

 can fall is about 1,350 feet (411 m.), with a least width 

 of 1,025 feet (312 m.) at Swift Drift point. The deepest 

 sounding by the U. S. Lake Survey was in the centre 

 opposite Prospect point, where a depth of 189 feet (57 -6m.) 

 was found, and Spencer's deepest sounding, below Goat 

 island near the Horseshoe fall, was 192 feet (58 -5m.). The 

 average central depth north of the American fall is not 

 far from 160 feet (48-8 m.), but there are four soundings 

 in that stretch which show 186 feet (56-7 m.). Above the 

 American fall the central depth is 100 to 120 feet (30-5 to 

 36-6 m.). This section is the correlative of the present 

 stage of the great lakes, and is still in the making. 



The gorge characters and sections and their correlatives 

 in the lake history are summarized in the following table: 



