Sedimentary Rock

Almost all fossils are found in sedimentary rock. But just what are sedimentary rocks? There are three major kinds of rock: Igneous, Metamorphic and Sedimentary. Igneous rock, such as granite and basalt, form as magma crystallizes. Metamorphic rocks, like quartzite and slate, form as heat, pressure or chemical activity alter pre-existing rocks.

Sedimentary rocks are formed by the consolidation of already existing particles or through chemical processes. Gravel, sand, mud and plants all contribute to the making of sedimentary rock. Limestone, gypsum and potash are chemically formed sedimentary rock.

Geologic processes at work now have acted in a similar manner throughout time. This principle is known as uniformitarianism. It is the cornerstone of the Earth sciences, including palaeontology. "...no vestige of a beginning, no prospect of an end" So ends an essay written by James Hutton in 1785, the first account of uniformitarianism. In his essay, Hutton attempted to describe how the Earth operated through time.

Hutton studied law, medicine and agriculture before devoting himself to scientific interests. His Theory of the Earth, published in 1795 provided massive evidence for the theory of uniformitarianism and gave fundamental shape to modern geology.

Hutton's Earth was a self-repairing machine. As wind and water gradually eroded the continents, debris was carried down to the sea where it accumulated, layer upon layer, on the sea floor. The layers were compressed into rock and the rock eventually formed new continents.

This concept of slow transformation occurring over long period of geological time provided the framework upon which evolutionary theory was constructed. Just as the Earth undergoes gradual change, so do the plants and animals that live upon it. The changes always happen within the constraints provided by known processes.

Erosion and deposition are age-old processes. Over long periods of time they have created the present landscape.

In the Red Deer River Valley, initially eroded by melt water from glaciers, the hillsides are cut into graceful, eerie badlands. Particles of soil and stone are loosened and carried to the river.

The river transports sediments to the sea. Some sediments are deposited along the route, but others are picked up and deposited farther downstream.

Layers of sedimentary rocks appear on each side of this valley near Drumheller. These layers were once continuous horizontal layers. Long after they were deposited, a stream cut through the strata, leaving them exposed on both sides of the channel.

One Thing After Another

In order to understand Earth's history, we need to place geological events in their correct order. Relative positions of sedimentary rocks layers help determine this order.

Sometimes, however, the original order is disturbed. The sediments pictured here were originally laid deposited layer upon layer on an ocean floor. About 40 to 50 million years ago, great stresses in the Earth heaved these ancient layers upwards. Erosion has been transforming them ever since.

This document was prepared by Wayne Hortensius, Calgary, Alberta, Canada for the Royal Tyrrell Museum Cooperating Society. All information © 1995 Royal Tyrrell Museum. All Rights Reserved.
Updated: April 8, 1995

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