LIFE OF THE QUATERNARY PERIOD. 575 



Glacial lakes are formed in several ways : (a) They may be rock- 

 basins scooped oat by glaciers where the rocks are softer or else where 

 there is a sudden change in the slope of the bed from a higher to a 

 lesser angle ; or (b) they may be formed by the damming of drainage 

 waters behind terminal moraines of a retreating glacier ; or (c) by the 

 disappearance of snow from old cirques, the fountains of ancient 

 glaciers. These three kinds are very abundant in all the highest 

 mountains, such as the Sierra or the Colorado; the last among the 

 highest summits, the first high up the valleys, and the second a little 

 way down. Again (d) along northern coasts elongated lakes are 

 often formed by the elevation of fiords. Many lakes in Norway 

 and Scotland are formed in this way; (e) lastly, the thousands of 

 small lakes which over-sprinkle the surface left by the ice-sheet, es- 

 pecially after its second advance, are due to irregular dumping of gla- 

 cial debris. 



It is necessary to remember that lakes are ephemeral features of 

 topography. They are inevitably in time either drained away by down- 

 cutting of their outlets, or else filled up by sediments brought down 

 from above. This process is especially rapid in mountain-regions. 

 The little glacial lakes are rapidly being filled and converted into 

 marshes and meadows. Everywhere in the Sierra, the region of mead- 

 ows is the region of the old glaciers. Lakes, then — especially small 

 lakes — are a feature of neiv topography. The topography of all the 

 Southern States is extremely old, while that of the Northern States has 

 been largely determined by the Drift, and is therefore very new. 



Life of the Quaternary Period. 



Plants and Invertebrates. — Remains of the life of the Quaternary, 

 both animal and vegetable, are very numerous, and often very well pre- 

 served. Both the plants and the invertebrate animals are almost wholly 

 identical with those now living on the earth. We will therefore dis- 

 miss these with one important remark : The plants and the marine 

 shells show an arctic climate in now temjjerate regions. The species 

 found are still living, but living farther north. There has been a mi- 

 gration of species northward since Glacial times. In Tertiary times 

 (p. 507), we noted a migration of forms southward, indicating a con- 

 trary change of climate at that time. 



Mammals. — But the mammalian fauna of the Quaternary is almost 

 wholly peculiar. It differs greatly from the Tertiary fauna preceding, 

 and the present fauna succeeding. The species are, moreover, very 

 numerous, and many of them of extraordinary size ; for it is the culmi- 

 nation of the mammalian age. It is necessary, therefore, to describe 

 some of them, and the conditions under which they were preserved^ 

 and thus to realize in some degree the conditions under which they 



