370 PRACTICAL LESSONS IN SCIENCE. 



time at least 200 feet above the present level of Lake Erie. Other 

 beaches, at lower levels, seem to indicate that the barrier was cut 

 down at an irregular rate. 



Icebergs were doubtless common on this inland sea, and per- 

 haps on other lakes of this period. It is thought by many that 

 they had much to do with distributing glacial material near the 

 margin of the drift area in the southwest, as there is often no evi- 

 dence of glacial ice having reached that region. 



The Champlain period of Europe was much the same as in 

 North America, and is often called the era of fresh water for- 

 mations. 



The glacial period merged gradually into the Champlain, and 

 the Champlain as gradually into the terrace period. The north- 

 ern regions began to rise again, sluggish rivers became narrower 

 and more rapid, other rivers were formed, lakes and inland seas 

 were drained away, carrying along great quantities of fine silts 

 and sands ; and at length the slender, rapid rivers began cutting 

 out channels in the sediments that filled the older, deeper, 

 wider valley. In this way bluffs and terraces were formed which 

 gave name to the period. Sometimes several terraces have been 

 formed, which are often known in common language as first bot- 

 tom, second bottom, etc. 



Where the drainage was not complete, extensive marshes or 

 wet prairies and multitudes of little ponds or lakes are the only 

 remains of wider bodies of water. 



The life of the quaternary was interesting as being the culmi- 

 nation of the mammalian type. 



The plants and marine mollusca that during the glacial period 

 lived in what are now temperate regions, are represented at the 

 present time by species living in polar or sub-polar climates. As 

 the ice retreated both plants and mollusca migrated northward. 

 Very little is known about the birds, fishes and reptiles of the 

 period, but it is supposed that there was not much change, sim- 

 ply a migration southward as the ice advanced and a migration 

 northward as the ice retreated. 



The mammalia of this period were abundant, varied in form 



