When Laurentia united with Baltica, island arcs in a proto–Pacific Ocean
called the Panthalassa began to collide with the western margin of what is
now North America.The collisions led to the Antler orogeny, which intensely
deformed rocks in the Great Basin region from the California-Nevada border
to Idaho (Fig. 88). It is named for Antler Peak near Battle Mountain, Nevada.
It consists of widespread, coarse clastic sediments and a prominent eastward-
directed thrust fault known as the Roberts Mountain Thrust.
Laurasia occupied the Northern Hemisphere, while its counterpart
Gondwana inhabited the Southern Hemisphere. Much of Gondwana was
in the southern polar region from the Cambrian to the Silurian. The pre-
sent continent of Australia sat on the equator at the northern edge of
Gondwana. A large body of water called the Tethys Sea, named for the
mother of the seas in Greek mythology, separated the two supercontinents.
Evidence for the existence of a wide seaway between the landmasses comes
from a unique specimen of flora called Glossopteris(Fig. 89) found in the
southern lands but absent in Laurasia.The Tethys held thick deposits of sed-
iments washed off the continents.Figure 88The Crag
Lake graben formed by
block faulting in Idaho.
(Photo by H.T. Stearns,
courtesy USGS)
Historical Geology