MESOZOIC AND CENOZOIC GEOGRAPHY 473 
been formed as such valleys are known to be, by action in the 
air. Granting this, then the conclusion must be that these parts 
of the sea bottom, whose depth in places is as great as one thou- 
sand feet below sea-level, must once have been exposed to the air. 
After this there came another depression, in which 
a part of the present coast-line, especially of the 
southern states, was lowered below sea-level. This 
involved particularly the Florida peninsula and the 
Gulf coast, where an arm of the Gulf imvaded the 
Mississippi valley for a distance of several hundred 
miles. 
The climate of the Tertiary period was warm. It was 
so much more moderate than the present, that plants, 
like the willow, maple, and other trees characteristic of 
the temperate zone, lived on the seashore as far north 
as Arctic exploration has gone. Fossils of these plants 
may be taken out of the rocks from beneath permanent 
cliffs of ice. Hence during this period, a temperate 
climate existed where now permanent ice covers the 
land. Toward the close of the Tertiary, and accom- 
panying the great uplift that has just been described, 
there came a change in climate, from the warm, equable 
condition of the Tertiary, to cold Arctic conditions. 
This continued into the next era, the Quaternary, and 
developed into the Glacial period. This change in 
climate not only accompanied the uplift, but perhaps 
was caused by it. 
