606 METASPERMAE OF THE MINNESOTA VALLEY. 
sufiiced 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 distinguished. 
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 
