MESOZOIC AND CENOZOIC GEOGRAPHY 475 
This arid condition which now forbids the filling of 
the lake basins with water, has replaced more moist 
conditions in Quaternary times. At first, in the early 
Quaternary or later Tertiary times, these basins were 
dry, as at present. Then the waters rose as the cli- 
mate became more moist. This was: followed by 
another period of aridity, when the basins lost much, 
if not all, of their water supply. Following this came 
another period of moisture, when fresh-water lakes were 
again present; and now we find the basins again dry. 
These changes in lake history, and hence also in 
climatic conditions, are definitely recorded on the 
margins of many of the lake basins of the west, in 
the form of ancient beaches and wave-cut cliffs. 
‘These records are so clear that even the most un- 
‘educated must notice them. 
The Glacial Period.'— Cause. The change in climate, 
from the warmth of early Tertiary to the Arctic con- 
ditions at its close, finally succeeded in enwrapping in a 
great ice sheet, both northeastern North America and 
northwestern Europe (Fig. 264). There seems to have 
been little or no ice in Asia, and very little in north- 
western America. The explanation for this change in 
climate has been thought by some to be astronomical. 
Through changes in the relative position of sun and 
earth, there is a variation in the distribution of heat 
1 See Chapter XI., Glaciers. 
