comprising a sequence of late Paleozoic tillites and coal beds, extended over
an area of several thousand square miles and reached a total thickness of
20,000 feet. Among the coal beds, the best in Africa, are fossil Glossopteris
leaves.Their existence on the southern continents is among the best evidence
for the theory of continental drift.
The glaciation was probably caused by the loss of atmospheric carbon
dioxide.The burial of carbon dioxide in the crust might have been the key to
the onset of all major ice ages since life evolved on Earth. Substantial carbon
dioxide repositories during the latter part of the Paleozoic were the great
forests that spread over the land. Plants invaded the land and extended to all
parts of the world beginning about 450 million years ago. Lush forests that
grew during the Carboniferous stored large quantities of carbon in their
woody tissues. Burial under layers of sediment compacted the vegetative mat-
ter and converted it into thick seams of coal (Fig. 114). The reduction of the
carbon dioxide content in the atmosphere severely weakened the greenhouse
effect, causing the climate to cool.
The continental margins became less extensive and narrower, confining
marine habitats to nearshore regions. This might have influenced the great
extinction at the end of the Paleozoic. No major extinction event occurred
during the widespread Carboniferous glaciation about 330 million years ago,
Figure 114A thick coal
bed at Little Powder
River coal field, Montana.
(Photo by C. E. Dobbin,
courtesy USGS)CARBONIFEROUS AMPHIBIANS