INCLINATIOi!^ OF BEACHES. 235 



present altitude, and the total amount of the differential elevation in this 

 extent of 300 miles, after subtracting- a quarter part for the probable or 

 possible effect of ice attraction, was about 200 feet. 



Alternate stages of elevation and rest. — The considerable number of defi- 

 nite additional shore-lines observed in proceeding to the north indicates, like 

 the stages of erosion by the River Warren between the times of formation 

 of the beaches near Lake Traverse, that there were periods of compara- 

 tively rai)id iqjlifting which alternated with others of repose or of very slow 

 progress of the general epeirogenic movement. Vertical uplifts of 10 to 

 20 or 25 feet were many times re^ieated, and were separated by longer 

 intervals of rest. But the initiation of these stages of uplift was delayed 

 until long after Lake Agassiz began to exist. The ice-sheet had retreated 

 from Lake Traverse to Manitoba, and three or four conspicuous moraines 

 of recession had been formed, while the lake level reposed undisturbed 

 during the formation of the first and highest beach of the Herman series 

 and the accumulation of the contemporaneous deltas. Such tardiness in 

 the beginning of the elevation, of this area, as it is recorded by the inclined 

 shore-lines, implies, and, indeed, makes it almost certain, that very little 

 uplifting, if any, had taken place during the time of melting away of the 

 ice above. If the restoration of the land to its wonted height had already 

 begun under the thinned edge of the ice, it would probably have gone 

 forward more prom])tly, while the Red River Valley was being gradually 

 occupied by Lake Agassiz, following- upon the retreat of the ice-front. 



Later and greater inclination of beaches along the base of Riding and Duck 

 mountains. — On the area of 300 miles extent from south to north between 

 the mouth of the lake and Gladstone, the epeirogenic differential uplift was 

 mo.sth done before the times of formation of the Campbell and McCauley- 

 ville beaches, the last two belonging to the southward outlet; but farther to 

 the north, within the area at the base of the escarpment of Riding and Duck 

 momitains, where Mr. Tyrrell has mapped tlie beaches of this lake and 

 determined their heights, a very important differential elevation, amounting 

 to alxiut 3 feet per mile along a distance of 50 miles between Valley and 

 Duck rivers — that is, between latitudes 51° 15' and 52° north — took place 

 after the Campbell and McCauleyville beaches were formed, since they ai'e 



