Theory of geologic periods

Stan in front of the Ordovician rocks of Bell Island

The Ordovician rocks of Bell Island represent the only rocks of that period on the Avalon Peninsula in Newfoundland. They date from a time of 490 to 445 million years ago.

The established geologic periods were not just arbitrarily set but represent distinct events in earth's past. Most of the time period boundaries are set based on the appearance or disappearance of various life forms. The Ordivician began after a major extinction event that ended the Cambrian period (trilobites predominate) and ended with another mass extinction when 60% of marine life was wiped out.

You can see in the picture the typical layering common to sedimentary rocks. These rocks are made up of fine grained shales and somewhat coarser sandstones. If you imagine the layers as a deck of cards, the deck would be tilted from left to right and dip slightly away as if you were looking under the deck to see what the last card was. The whole sequense of rocks are estimated to be some 1,200 metres thick.

Oolitic hematite (don't ya like that word!) beds were laid down higher up in the sequence but they can't be seen here because the rocks dip away from this view. They outcrop several miles inland where they were mined for iron ore over a period of 70 years.

The most interesting thing about these rocks are the wormcastings that can be seen on some of the rock surfaces when they break apart. The wormcastings show up as trails in the rocks where worms burrowed 100's of millions of years ago. That really puts time in perspective if you think time flies now. In the grand scheme of things though its only the blink of a geologic eye!

Tony :-)