606 METASPERMAE OF THE MINNESOTA VALLEY. 



sufficed for the development of a characteristic flora. Then 

 the second great southward movement of the ice began, during, 

 and perhaps late in which, the moraine of the Lake Region of 

 Minnesota was deposited and the debris piled up in the Leaf 

 hills to a level of three hundred and fifty feet above the sur- 

 rounding country. The length of time that this glacier 

 persisted in its southern extension is not known, but since its 

 recession it has been calculated by Winchell from a study of 

 the gorge of the Mississippi from St. Anthony Falls to Fort 

 Snelling, and of the observed rate of recession of the falls, 

 that a period of 7,800 years has elapsed. It is not certain that 

 the proximity of the glacier even at its intermediate extension 

 of the lake- region moraine would have prevented a plant 

 population from establishing and maintaining itself in the 

 valley of the Minnesota. To-day, in the Alps, one finds flowers 

 blooming within a few feet of a glacier, and in Alaska it has 

 been observed that plants of even a large size may continue 

 their growth upon a slowly moving moraine. It is probable, 

 however, that the adjacency of so large a body of ice, through 

 its influence upon humidity and temperature, had an indirect 

 influence upon the physiognomy of the Minnesota plant- 

 population. 



Results of the epoch of glaciation. The results of this 

 widespread glaciation of the northern portion of the North 

 American continent, in its effect upon plant-population in the 

 Minnesota valley from the time of glacial retreat to the present, 

 may be classified under two general categories. First, the 

 effects of the glaciation upon the soil, topography and climate 

 of the valley itself must be noted, and, second, the effects of 

 the glaciation upon the plants, in so far as concerns modifica- 

 tions of types or novelties of distribution or habitats, are to 

 be distingaished. 



Under the first division of the subject the most important 

 result is doubtless the great mixing of soil-components so as 

 to form the characteristic clays, sands or gravels of the till. 

 Since a large sheet of Cretaceous deposits was torn from the 

 surface of the older rocks by the energy of the glacial advance 

 the subsoils of the till region contain considerable of the Cre- 

 taceous elements. They are rather rich in calcareous, 

 magnesian and silicious elements. The thorough kneading 

 together of the various constituents has produced a soil some- 

 what generalised in its chemical character, and this soil by 

 subsequent modifications presents from place to place a wide 



