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Landforms

What do rock formations tell us about changes on Earth?


To answer your question I have chosen an example that are commonly observed in southern Canada around the margins of the Canadian Shield. The Shield is Precambrian (greater than 570 million years old) in age, in fact, some of the oldest rocks in the world (3.6 billion years old) have been discovered in the Canadian Shield. The Shield consists of granite, gniess (granite that has undergone metamorphism) and greenstone (a type of volcanic rock). These old rocks formed the earliest continents. Over a long period of time, these rocks were subjected to weathering and erosion; this produced a gently rolling landscape covered by a considerable thickness of weathered rock (geologists refer to this as regolith).

In Cambrian times the sea level began to rise and the old continents were flooded by the oceans. Waves washing on the shores of the old continents worked the regolith into beach sand. These beach sands were deposited as a thin layer of flat-lying sandstone over the former rolling surface of the continents. As sea level continued to rise throughout the Early Paleozoic period (570-350 million years ago) different types of sediments were deposited. At this time in geologic history much of North America was situated close to the Equator. In the shallow tropical seas that covered North America, coral reefs flourished and thick beds of lime mud were deposited on the sea floor. These shallow extended to cover all of the present land area of Canada. The limestone rocks that were deposited in these Early Paleozoic seas have considerable economic importance today. Much of the oil that is discovered in Canada comes from these old limestone rocks!

Alec Aitken, Department of Geological Science, University of Saskatchewan


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    Last Modified: 2004-12-10