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. |
