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HISTORY
  Big Bang
  Galaxies etc.
  The Earth
  Continents
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  Precambrian
  Paleozoic
  Mesozoic
  Cenozoic
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Precambrian

© 1985 by Andrew J. Morris

The Precambrian time spans from when the Earth's crust cooled and solidified some four billion years ago, up to about 600 million years ago. The term itself is outdated, usually replaced with three eons: Hadean, Archean and Proterozoic, though sometimes called Protocambrian and/or Eocambrian. The land surface was lifeless during most of this time, though primitive life forms were plentiful in the oceans. Toward the end of the era some simple plants such as algaes and ferns may have begun to conquer the land, along with the simplest of animals such as snails and worms, first in intertidal areas and later further inland.

The surface of the continents during the early Precambrian consisted almost entirely of valcanic rocks. Almost as soon as they formed these surface stones were subjected to weathering, and so began to break down. Thermal expansion and contraction and chemical interaction with the atmosphere loosened tiny bits of rock from the surface, then winds and water carried these abrasive particles colliding into other rocks, to gradually wear them down. Meanwhile stellar dust and debris continued to rain down on the earth as well, from the tiniest specks to massive meteorites.

Rain and flowing water tended to carry the smaller particals to the oceans or into vast inland seas. In many cases these layers of tiny particles condensed under pressure and heat, turned back into solid rock, as might happen when volcanic lava preads over them, or they become so deeply buried under the other sediments that they were metamorphised. In other cases these sediments may have transformed into less densely compacted stone, such as sandstones, siltstones, shales, etc. These "new" rocks were in turn weathered down to tiny particles of sand and dust, or subjected to metamorphising pressure and heat, depending on the circumstances of their location.

About 2.5 billion years ago a chain of mountains formed where the Rocky Mountains now stand, pushed up by forces within the crust of the planet. A half billion years later they were no more than hills, if they existed at all. They had been worn away by those forces of erosion we just described. Then, about 1.5 billion years ago, another chain of mountains rose up in the same area, only to fall again to the persistent and inevitable force of erosion. By the end of the Precambrian era, Colorado was part of a vast flat plain, near sea level.

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