300 Glacial and Modified Drift 



twelve distinct and successive moraines mapped west and 

 north of this part of the Mississippi. 



Above the junction of the Minnesota river, the upper Mis- 

 sissippi passes through six moraines formed later than those 

 noted as confluent close south and east of St. Paul, these of 

 later dates being the Waconia, Dovre, Fergus Falls, Leaf Hills, 

 Itasca, and Mesabi moraines, which in this order are the sixth 

 to the eleventh of the Minnesota series. Only one, the last and 

 most northern recognized in the state, named the Vermilion 

 or twelfth moraine, runs through northern Minnesota .beyond 

 the Mississippi watershed. 



Details of the course, topographic features, material and 

 structure, and the chronologic sequence, of these most promi- 

 nent drift deposits of our region, have been published through- 

 out the many chapters describing our counties in the final re- 

 ports of the Minnesota geological survey. Little attention 

 was given there, however, to the very interesting question of 

 the probable length of time, in years and centuries or in thou- 

 sands of years, occupied by the accumulation of this numerous 

 series of frontal moraines, marking short or long pauses, or 

 sometimes re-advances, of the ice border during its general 

 wane and departure from the state area. 



But in another work, for the United States Geological 

 Survey, on the Glacial Lake Agassiz, I have shown reasons for 

 ascribing to the entire history of that vast ice-dammed lake^ 

 stretching gradually about seven hundred miles from south to 

 north in the valley of the Red river and the basin of lake Win- 

 nipeg, no longer time than one thousand years. This is a pro- 

 portional estimate, in connection with the evidence set forth by 

 N. H. Winchell, G. F. Wright, and other glaciaUsts,' both in 

 America and Europe, including the present Avriter, that the 

 Postglacial period, since the recession of the ice-sheets from 

 the northern L^nitcd States and Canada and from northwestern 

 Europe, measures about 10,000 to 5,000 years, being approxi- 

 mately alike on opposite sides of the Atlantic. 



In comparison with these estimates, the time required for 

 the formation of any one of our great marginal moraines could 

 be no more than a few decades. All the retreat of the ice- 

 sheet on the moraine-bearing region of Minnesota, from the 

 compound belt of marginal drift hills adjoining St. Paul to the 

 Itasca, Mesabi and Vermilion moraines, at and bevond the 



