PAUSES IN THE CRUSTAL UPLIFT. 499 



because of the decreasing attraction of the ice during the stages of its 

 retreat between these moraines. But the extent of the liighest beach 

 shows that no appreciable changes of level took place while the ice-sheet 

 was being melted back 250 miles or more and was probably nuich reduced 

 in thickness upon a large area farther north, meanwhile, at times of halt in 

 its recession, or perhaps of some readvance, accumulating the most massive 

 morainic deposits of this region. The stability of the crust had been main- 

 tained during this great reduction of the ice pressure ; and when at length 

 an uplift ensued, the process was slow and marked by no paroxysmal 

 action, but progressed in a gradual manner, though yet Avith pauses, as was 

 also doubtless the method of the continued retreat of the ice. 



Pauses in the crustal uplift recorded by the series of heaches. — The suc- 

 cessive beaches of Lake Agassiz, numbering seventeen in its northern part 

 while the lake outflowed southward by the river Warren, and fourteen 

 while it outflowed northeastward, appear to have been formed during 

 pauses in the differential elevation of this area. Between the times of 

 formation of the beaches the uplift of the laud was too rapid to be recorded 

 by wave erosion and beach deposits, and the definitely marked shore-lines 

 belong to stages of interruption or slower progress of this crustal uplift. 



At the southern end of the lake each of the beaches, into which several 

 in their coui'se from north to south become merged, may belong to a slowly 

 sinking lake surface, with change of 5 feet, or in some cases 10 feet, during 

 the accumulation of the single beach ridge; aud the intervals of 10 or 15 

 feet between the levels held by the mouth of the lake while the beaches of 

 its southern part were being formed appear to represent times of exception- 

 ally rapid erosion because of comparatively fast elevation of that area and 

 of the country crossed by the River Warren. Along the course of this 

 stream, the present valley of the Minnesota River, no outcropping rocks 

 are found at so high levels that they would be touched by the contiimatiou 

 of the plaues of the upper beaches of Lake Agassiz, having in theu* 

 southern part a descent to the south of about a half of a foot per mile. 

 The River Warren cut its channel wholly in glacial drift, until during the 

 McCauleyville stages of the lake it reached the ledges of granitoid gneiss 



