228 THE GLACIAL LAKE AGASSIZ. 



ranging from 1 inch per mile at the south to about 2 J inches per mile from 

 the international boundary to the latitude of Gladstone. 



With frirther exploration and study, including the portion of the lake 

 area examined by me in Manitoba, I became convinced that this explana- 

 tion is inapplicable to the problem, because the highest beach of the Her- 

 man series (formed contemporaneously with the six large deltas which were 

 dependent for their formation on the accompanying retreat of the ice-sheet 

 supplying their sand and gravel) is found to be continuous along an extent 

 of nearly 250 miles from south to north, reaching from Lake Traverse at 

 least to Thornhill, in Manitoba, across an area which has several prominent 

 moraines of recession, denoting important stages of decrease of the ice- 

 sheet. These moraines extend to the borders of Lake Agassiz, and the 

 ice front at the time of then- formation traversed the lake basin. There- 

 fore, if diminution of the ice-attraction were the principal cause of the 

 changes of the levels of the lake, we should expect the highest beach to 

 cease at the successive morainic belts, and another somewhat lower to take 

 its place thence northward. 



For aid in the investigation of this and other movements of elevation 

 of the land following the departure of ice-sheets and the evaporation of 

 Lake Bonneville, Mr. R. S. Woodward, of the. United States Geological 

 Survey, made a carefiil mathematical computation of the effects of such 

 masses of matter formerly existing upon portions of the earth's sui'face to 

 deform the geoid or level of the water of lakes or the sea.^ His result, 

 agreeing approximately with conclusions from similar computations by 

 European mathematicians and physicists, shows that the North American 

 ice-sheet, with its known area and its maximum probable thickness, would 

 be capable of drawing the level of Lake Agassiz upward to the north not 

 more than a quarter, or perhaps no more than an eighth or tenth, as much 

 as the ascent of the Herman beach. It is thus evident that we must look 

 to some other cause for explanation of these changes of level, and this is 

 found in a differential uplifting of the lake basin, increasing in amount 

 from south to north upon all the area where we have determined the 

 heights of the beaches. 



' U. S. Geol. Survey, Sixth Annual Reiiort. pp. 291-300 : and Bulletin No. 48, " On the form and posi- 

 tion of the sea-level." 



I 



