186 W. UPHAM MODIFIED DRIFT IN SAINT PAUL. 



tially eroded and removed by the river, cutting down its channel there 

 nearly to its present level, before the deposition of the valley drift at 

 Fort Snelling and northward. 



Gradually, as the border of the ice-sheet receded, the avenues of drain- 

 age from its melting were occupied by modified drift, deposited pro- 

 gressively along the valleys, earliest at the south and later northward, as 

 fast as the ice was melted back. The Late Glacial history of the vicinity 

 of Saint Paul, however, which we learn from its plateau gravel and sand, 

 leads me to think that the absence or extreme scantiness of modified 

 drift in the valley from Fort Snelling to the southeastern limit of this 

 city is due to lack of deposition there, rather than to removal by erosion. 



Before the time of the glacial lake Ham line, in which the modified 

 drift plateaus of Saint Paul were accumulated, the high level valley drift 

 on the southeast had been mostly deposited. It was speedily and deeply 

 channeled when the ice barrier holding that lake was melted through in 

 the southeast part of the city of Saint Paul, opening this great valley to 

 drain away the lake and to carry the waters of glacial melting and of 

 rains from the basins of the Minnesota river and the upper Mississippi. 

 Subsequently, while the ice-sheet was being melted away along the course 

 of the Mississippi above Saint Paul, the valley drift of gravel and sand 

 so amply developed in Minneapolis and northward was spread along this 

 valley. This coarse stratified drift appears to have reached, however, 

 only to the vicinity of Fort Snelling, and the same is true likewise of 

 the terrace gravel and sand of the Minnesota valley, although much fine 

 silt and clay were doubtless at the same time borne by the river floods 

 past Saint Paul and far southward. 



Plateaus of Modified Drift. 

 general character. 



The most remarkable feature in the glacial geology of the city of Saint 

 Paul consists in its deposits of modified drift at high levels, forming a 

 group of plateaus of gravel and sand, bordered by steep slopes which 

 rise to nearly flat plains 100 to 125 feet above the highest terraces repre- 

 senting the old flood-plain of the river valley. These plateau deposits 

 tell of a water level peculiar to this area. 



Several districts of New England have similar modified drift plateaus 

 which are found to record ancient water levels. In southeastern New 

 Hampshire, twenty years ago, I examined and mapped for the geolog- 

 ical survey of that state, under the direction of Professor C. H. Hitch- 

 cock, numerous plateaus of this class, which there were deposited by 

 streams discharged from the retreating ice-sheet and flowing into the 



