Introduction to Planet Earth - Chapter 20
-The construction of North America began about 4,000 million years ago with the
formation of the Acasta Gneiss of the North west Territories, which now forms
part of the Slave Province of the Canadian Shield.
-The building of North America was essentially complete by 65 million years ago,
although the modern landscape is the result of geologically recent glaciations that
have occurred in the last 2.5 million years.
-The last ice sheet left the southern portions of the country only 12,000 years ago,
and finally melted in Labrador 6,000 years ago.
-The northern part of the continent, in Canada, is underlain by the exposed part of
the ancient core of craton of North America; this exposed part is called
the Canadian Shield and consists predominantly of very
old, Archean and Proterozoic rocks. These rocks range in age from 4 - 1 billion years
old and are largely devoid of fossils.
-The craton is composed of a complex assemblage of several distinct
-Geologic Provinces are broad regions of similar rocks, usually covering many
thousands of square kilometers, with characteristics that differ significantly from
rock types present in adjacent areas.
-Individual geologic provinces have been subdivided into smaller units
called subprovinces, which are fault-bounded units containing similar rock types,
structures, and mineral deposits.
-Provinces and subprovinces are now widely recognized to be terranes.
-Terranes are discrete fragments of oceanic or continental material that have been
add to a craton at an active margin by accertion.
-These likely originated as small continents and remnants of ocean-floor crust, each
with its own complex geological history.
-Most of the North American craton was assembled between 1 and 4 billion years