a distinct trilobite species typical of North America but not of South Amer-
ica. The fossil evidence suggests that the two continents collided about 450
million years ago, creating an ancestral Appalachian range along eastern North
America and western South America long before the present Andes formed.
Later, the continents rifted apart, transferring a slice of land containing trilo-
bite fauna from North America to South America.THE CALEDONIAN OROGENY
As Laurentia approached Baltica at the end of the Silurian about 400 million
years ago, they closed off the Iapetus Sea some 200 million years before the
modern Atlantic began to open.When the continents collided, they crumpled
the crust and forced up mountain ranges at the point of impact. The sutures
joining the landmasses are preserved as eroded cores of ancient mountains
called orogens. Paleozoic continental collisions raised huge masses of rocks
into several folded mountain belts throughout the world. A major mountain
building episode from the Cambrian to the middle Ordovician deformed
areas between all continents comprising Gondwana, indicating their collision
during this interval.
Matching geologic provinces exist between South America, Africa,
Antarctica, Australia, and India. The Cape Mountains in South Africa have
counterparts with the Sierra Mountains south of Buenos Aires in Argentina.
Matches also exist between mountains in Canada, Scotland, and Norway. Dur-
ing this time, much of Gondwana was in the southern polar region, where
glaciers expanded across the continent, leading to an Ordovician ice age.
The closing of the Iapetus Sea from the middle Ordovician to the
Devonian as Laurentia approached Baltica resulted in the great Caledonian
orogeny (Fig. 75),or mountain building episode. This orogenic activity
formed a mountain belt that extended from southern Wales,spanned Scot-
land, and ran through Scandinavia and Greenland, possibly including today’s
extreme northwest Africa as well. In North America, this orogeny built a
mountain belt that extended from Alabama through Newfoundland and
reached as far west as Wisconsin and Iowa.Vermont still preserves the roots of
these ancient mountains, which were shoved upward between about 470 and
400 million years ago but have since planed off by erosion.
The middle Ordovician Taconian orogeny named for the Taconic Range
of eastern New York State culminated in a chain of folded mountains that
extended from Newfoundland through the Canadian Maritime Provinces and
NewEngland,reaching as far south as Alabama. During the Taconian distur-
bance, extensive volcanic activity occurred in Quebec and Newfoundland and
from Alabama to New York, extending as far west as Wisconsin and Iowa.Historical Geology