WA VE-LIKE PR O GRESS OF AN EPEIRO GEN I C UPLIFT. 389 



and unimportant oscillations, from that time until now. The 

 beginning or earliest known stage of the progressive elevatory 

 wave probably thus raised the northern half of the Mississippi 

 basin to a variable amount ranging from 100 feet or less to 500 

 feet or more. It was practically completed, for this area, previous 

 to the accumulation of the outer and earlier moraines in the series 

 of many which mark pauses in the further recession of the ice- 

 sheet. Thenceforward the glacial melting appears to have been 

 more rapid than before, giving to the ice steeper frontal gradients 

 whereby its drift was amassed more commonly in hills, ridges, 

 and lake-enclosing hollows, and especially in the very irregularly 

 knolly and hilly moraine belts. 



The rapidity of the glacial recession and of this ensuing 

 epeirogenic uplift .in its wave-like advance upon the area of the 

 glacial Lake Agassiz, extending nearly 700 miles from south to 

 north in the basin of the Red river and of Lake Winnipeg, sur- 

 passes all previous knowledge in what it reveals concerning the 

 mobility of the earth's crust. The postglacial duration of Lake 

 Michigan and its companion great lakes of the St. Lawrence 

 has been shown, by numerous independent but well agreeing 

 observations and estimates, to be no longer than 6,000 to 10,000 

 years. Now the amount of wave erosion on the shores of Lake 

 Michigan and the resulting accumulation of beach sand, heaped 

 into dunes upon large areas about the south end of the lake, 

 must exceed, by a ratio of 10 : i or 20" i, the corresponding 

 wave action in its total amount at all the successive levels held 

 by Lake Agassiz during its history, which accordingly must be 

 comprised within some such time as 1,000 years or perhaps less.^ 

 During this geologically very short time, the ice was melted 

 away upon the distance of 700 to 1,000 miles from the middle 

 of the west side of Minnesota to James and Hudson bays, and 

 the Lake Agassiz basin was differentially uplifted mostly 300 

 to 500 feet, to the height which it has ever since retained with- 

 out appreciable later change. To understand the wave-like devel- 



' Geol. and Nat. Hist. Survey of Canada, An. Rep., new series, Vol. 4, for 1888- 

 89, pp. 50, 51 E. 



