224 THE GLACIAL LAKE AGASSIZ. 



its bed to this additional deptli. Through a comparatively loug time, rep- 

 resented by the Herman beach, this large outflowing river, bearing the 

 waters supplied bj* the progressive glacial melting upon a vast area, had 

 only deepened its channel slightly; but at the close of this stage the di\'i- 

 siou between it and the next following Norcross stage, though doubtless 

 only a short interval of time, was marked by a considerable increase of 

 depth of the channel. Why was the river able to erode so much faster 

 then than during the time of formation of the Herman beach, or of the 

 Norcross beach afterward, which likewise rejaresents a nearly stationary 

 period in the progress of erosion of the Lake Traverse Valley I The 

 answer which seems best was suggested to me by Mr. G. K. Gilbert in 

 a letter dated February 3, 1888, as follows: 



* * * Retreat of the ice modified tlie geoid, and perhaps produced also a 

 crustal change, and in consequence the baselevel assumed a new attitude to the land. 

 The river adjusted its grade to the new conditions, and then remained stationary dur- 

 ing the formation of the Xorcross beach. 



The portion of Mr. Gilbert's explanation which we must appeal to is 

 that attributing the temporarilj- rapid erosioa to a crustal change, that is, to 

 an uplifting of the region about the mouth of Lake Agassiz; and this meets 

 the case fully. There was, however, no apparent reason why the region of 

 Lake Traverse or the Minnesota Valley should be thus intermittently ele- 

 vated, so far as we can directlj' compare the change with the process of the 

 glacial retreat; and to what extent this movement affected the northern 

 portions of the lake area can only be ascertained by very exact compari- 

 son of the altitudes of the lowest Herman and the highest Norcross beaches. 



Ehvthmic stages of elevation of the country across which the River 

 WaiTen flowed, intervening with pauses in the action of the uplifting forces, 

 are shown in succession ])y the Norcross beach, to which the erosion from 

 the level of the lake at the later part of its formation of the Herman beach 

 was about 20 feet; by the two Tintah beaches, to the first of which there 

 was fiu^ther erosion of about 15 feet, and a similar amount more to the sec- 

 ond; b}' the Campbell beach, to which again the river still further cut down 

 its channel 15 or 20 feet; and by the McCauleyville beach, formed by the 

 lake when its channel of outlet was the bed of Lake Traverse, once moi-e 



