4 Minnesota Plant Life, 



ment of ice took place there must have been a great modifica- 

 tion of drainage conditions over all the invaded district. Streams 

 were dammed, hills were levelled, valleys were filled, lake bot- 

 toms were hollowed out or covered with confused masses of 

 rocks and clay ground into pow^der by the powerful action of 

 the ice, continuing, as it did, through more than a thousand 

 years. It is clear, too, that the ancient vegetation must have 

 been almost wholly swept away by this invasion from the Arctic 

 zone. It is true that plants are sometimes found growing close 

 to the edge of glaciers in the Alps, in Greenland and in Alaska. 

 Sometimes even masses of soil are so borne upon the surface of 

 the glacier that plants of hardy habit may continue their exist- 

 ence there. Yet, with a due regard to these well-known facts, 

 it is not conceivable that for so long a period of rigorous cold 

 the old pre-glacial plant-population of the state could have held 

 its ground. It must be supposed, rather, that as the glacier 

 steadily advanced from the north, year after year plants flung 

 their seeds into air-currents moving southward or attached them 

 to the fur of animals seeking a warmer clime and thus gradu- 

 ally season by season themselves migrated toward the south. 

 Evidently those plants provided with seeds, buoyant, winged, 

 barbed or hooked were best fitted by such contrivances to leave 

 the snows and ice of a thousand years, while the plants with 

 smooth and heavy seeds either migrated more leisurely and 

 more sparingly or were quite extinguished by the cold. 



When the glacial period came finally to an end and the ice- 

 sheet moved north beyond the confines of the state, there 

 opened to the immigrants from the south a new Minnesota. 

 Great lakes formed by the waters of the melting ice now lay 

 where before there was land. Rivers were flowing in new direc- 

 tions and were carving for themselves new gorges through the 

 rocks. A fertile soil was deposited upon the hill-sides not, 

 indeed, a rich leaf-mould, but capable of supporting many kinds 

 of plants. Into this land of promise the southern plants began 

 to come. Winds from the south, animals ranging toward the 

 north and water-fowl in their annual migrations, brought back 

 in some instances no doubt the very same varieties which hun- 

 dreds of years earlier had fled before the ice, and in others, new 

 kinds born and bred in the south and seeking new homes where 



