Historical Geology Understanding Our Planet\'s Past

variations preceded changes in the extent of the more recent ice ages, suggest-

ing that earlier glacial epochs might have been similarly affected.The variations

of carbon dioxide levels might not be the sole cause of glaciation. However,

when combined with other processes, such as variations in Earth’s orbital

motions or a drop in solar radiation, they could become a strong influence.

Continental movements might also be responsible for the late Ordovi-

cian glaciation. Magnetic orientations in rocks from many parts of the world

indicate the positions of continents relative to the magnetic poles at various

times in Earth history. Paleomagnetic studies in Africa revealed very curious

findings, however.They placed North Africa directly over the South Pole dur-

ing the Ordovician, which led to worldwide glaciation.

Additional evidence for such widespread glaciation came from another

surprising location—the middle of the Sahara Desert. Geologists exploring for

petroleum in the region stumbled upon a series of giant grooves cut into the

underlying strata by glaciers. Rocks embedded at the base of glaciers scoured

the landscape as the ice sheets moved back and forth. Other collaborating evi-

dence suggests that thick sheets of ice blanketed the Sahara and included

erratic boulders placed by moving ice and eskers, which are sinuous sand

deposits from glacial outwash streams.

A major mountain building episode from the Cambrian to the Ordovi-

cian deformed areas between all continents comprising the southern super-

continent Gondwana, indicating their collision during this interval. The

Figure 67 Intensely
folded Cambrian
carbonate rocks of
Scapegoat Mountain,
Lewis and Clark County,
Montana.
(Photo by M. R. Mudge,
courtesy USGS)

Historical Geology