The continents were lowered by erosion. Shallow seas flooded inland,
covering more than half the present land area. The weight of the sediments
formed a deep depression in the ocean crust called a geosyncline. The sedi-
mentary rocks were later uplifted into great mountain belts surrounding the
Mediterranean Sea when Africa slammed into Eurasia. The inland seas and
wide continental margins along with a stable environment provided excellent
growing conditions and enabled marine life to flourish and spread through-
out the world.
During the late Silurian, Gondwana wandered into the southern polar
region around 400 million years ago and acquired a thick sheet of ice. Glacial
centers expanded in all directions. Ice sheets covered large portions of east
central South America, South Africa, India, Australia, and Antarctica (Fig. 90).
During the early part of the glaciation, the maximum glacial effects occurred
in South America and South Africa. Later, the chief glacial centers switched to
Australia and Antarctica, providing strong evidence that the southern conti-
nents wandered locked together over the South Pole.
In Australia, Silurian-age marine sediments were found interbedded
with glacial deposits. Tillites, composed of glacially deposited boulders and
clay, were separated by seams of coal.These indicate that periods of glaciation
were punctuated by warm interglacial spells when extensive forests grew.The
Karroo Series in South Africa is composed of a sequence of late Paleozoic lava
Figure 89Fossil
Glossopterisleaf, whose
existence on the southern
continents is strong
evidence for Gondwana.
(Photo by D. L. Schmidt,
courtesy USGS)SILURIAN PLANTS