No. 5.] SPENCER — SURFACE GEOLOGY. 283 



sent surface of the land where the ancient valleys are enth'ely 

 obscured. At the same time the erosive effects were obscured 

 by the stones and dehria deposited by the melting glacier, being 

 transported by the waters rushing down the steep pitch of the 

 river beds. With an increased elevation of the laud, the conti- 

 nent would be more elevated to the northward, which would still 

 further increase the velocity of the waters flowing southward, and 

 retard or altogether stop those flowing northward. Other exca- 

 vating eftects would be produced by the glaciers shoving for- 

 ward the decomposed rock beneath tliemselves. The existing 

 valleys would to a greater or less degree determine the direction 

 of the glacier itself. These glaciers, laden with stones and debris, 

 moving over the land would naturally plane off the rocks below 

 them, and the stones and sand contained in the ice would pro- 

 duce their striated and polished surfaces. The glaciers would 

 transport the local material by the thrusts ; and the rocks and 

 other contained debris derived from the source of the glacier itself 

 would be deposited as it melted, tlius producing terminal (and 

 also lateral) moraines. In order that the glacier could move 

 southward it is not necessary that the surface of the land should 

 have any slope, for if the ice were sufficiently deep, the weight to 

 the northward or towards its source, would cause it to flow like 

 a mass of apparently solid pitch, which when piled up is con- 

 stantly seeking a lower level. CroU has calculated that the ice 

 could flow if the surface stood at half of one degree above the 

 ocean level. The terminal moraines produced would tend to dam 

 the waters beneath the glaciers caused by their melting. 



After the erosion by glaciers (and the striations of the surfaces 

 of the rocks) was accomplished the continent began to be de- 

 pressed, and the subsidence went on until the land was more than 

 50(1 feet below the present altitude. (But we will subsequently 

 see that the depression continued till a submergence of 1800 

 feet at least, or perhaps several times that depression was attained). 

 This subsidence and also the previous damming of lake and river 

 basins produced immense inland lakes beneath the continental 

 glaciers, or floating icebergs derived from them. As the glaciers 

 melted, the transported debris contained in them was deposited in 

 an unstratified manner on the land, or where it fell into water it 

 was partly stratified. This period of the glacier constitutes the 

 Diluvian era or Lower Champlain epoch. The preceding period 

 of elevated continent forms the period of glacial diift. But the 



