In the beginning, there was no eastern coast of North
America, although we have shown its modern outline to illustrate
how the earliest lands came to relate to it. The blued areas were
then oceanic, and the today's extra real estate was rafted into place
from other parts of the globe. The process by which they became joined
to this old land mass has come to be called continental drift.
When the planet was formed 4.5 billion years ago, dense matter
collected at the coreof the earth while less dense material solidified
as rock at the surface. These "cratons", or proto-continents,
accreted to one another as the nucleus of present-day North America.
The oldest pieces of crust, dating from about 4 billion years in the
past, are found in the Slave Craton (1). The Rae Craton (2), the
Hearne(3) and the (4) Wyoming are also of Archaen age, as is the
(5) Superior Craton (2.5 billion years).
From about 2.1 billion years ago, during the Proterozoic Era of time,
oceans opened and closed between these ancient cratons, Islands and
other masses of crust were squeezed between them, and mountain chains
arose at some of these points of collision. The exposed remains
of all this activity in Precambrian time is termed the Canadian or
Precambrian Shield.
A collision with an early prto-continent coming in from the ancient
ocean (arrow) gave rise to the planet's mightiest mountain range (light
tan). This was the 5,000 kilometre-long Grenville Chain. In all
of this time, the margins of present-day North America were thinly
crusted oceanic basins, flooded with salt water and recipients of the
products of erosion and weathering from the interior highlands. Any
life that developed was restricted to the ocean.
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