320 THE ICE AGE IN NORTH AMERICA. 



Another striking deposit of this final glacial drainage can 

 be seen on the eastern flank of the Adirondack Mountains 

 at Chazy, a few miles west of Plattsburgh, where there is a 

 large and most remarkable accumulation of rolled river 

 pebbles two or three hundred feet above the level of Lake 

 Champlain, while there is no stream bed anywhere near 

 this locality. The only explanation of it is (and it is entirely 

 adequate), that this was a line of temporary drainage for the 

 glacial floods which for a time accumulated in the upper 

 St. Lawrence Valley and found their only passage for a while 

 between the mountain mass of the Adirondacks and the wan- 

 ing ice which filled the Champlain Valley. (For an illustra- 

 tion from Alaska, see Plate VIII.) 



From the Missouri Valley there come some of the most 

 startling facts revealing the extent of the floods which charac- 

 terized the later stages of the glacial period. The present 

 Missouri discharges (according to Humphreys and Abbott) 

 twenty-nine cubic miles of water annually. Yet, even so, 

 there are frequently floods in the lower part of the valley 

 which reach a height of thirty or forty feet above present 

 low water mark. But at the time of the greatest extent of 

 the ice-sheet the drainage of not far from 250,000 square 

 miles of ice covered area found its way into the Middle Mis- 

 souri. For a considerable time towards the close of the 

 glacial epoch the ablation of this ice would, at a moderate 

 estimate, amount to ten feet per annum over this whole 

 contributory surface. This would furnish 500 cubic miles of 

 additional water to be carried off through the lower portion 

 of the river channel during each summer between April and 

 November. An examination of the lower channel shows that 

 at Hermon twenty-five miles below the mouth of the Osage 

 River where it joins the Missouri the passage between rocky 

 bluffs 300 feet high is barely two miles wide. Mathematical 

 calculations will show that it would require ninety-six days 

 for a current two miles wide and two hundred feet deep, flow- 





