Canadian Rockhound: Sedimentary Rocks

About Us | Contact Us | Services | Copyright

Sedimentary rocks are formed from sediments, just as the name says. Only 5 per cent of the Earth's crust consists of sedimentary rocks, yet 75 per cent of the rocks exposed at the Earth's surface are sedimentary. Weathering wears down the other rocks into silt and rain and rivers wash the silt down and deposit it in layers. That is one way to identify sedimentary rocks – you can see layers in the rock (Figure 1). Most fossils are found in sedimentary rock – in sandstone and coal for example.

Figure 1. Wasson's Bluff, Nova Scotia. Notice the layers of sediments.

Weathering

Rocks at the Earth's surface are subject to weathering by the chemical action of acid and alkali salts in rain and ground water. They are also broken down mechanically. Expanding ice, and alternate heating and cooling crack the rocks. This allows water to get in the cracks and when the water freezes in winter the expanding force is so great that the rock can shatter.

Plant roots break up the surface of a rock allowing water to penetrate the cracks. This water helps the chemical breakdown of rocks. Frost shattering is a major processes of mechanical breakdown and is responsible for the accumulation of angular rock fragments, known as scree deposits, at the foot of steep mountain slopes.

In hot deserts, rocks can fracture as they expand during the heat of the day and contract during the cold night.

Tunnel walls in a mine can collapse with explosive force because of the pressure of the surrounding rock. This happens to some extent at the Earth's surface. Many rocks that are now exposed were once buried under great masses of overlying layers of rock. Once the overlying rocks have been removed by erosion, the release of pressure can cause fractures parallel to the rock surface. These fractures are known as sheet joints.

Transportation

Once the rock has been weathered, the material – silt – may be moved by transport. This is not a big truck! It can be a stream of water that can transport undissolved particles by bouncing them along the stream bed. This is known as saltation; if the particles are small enough, they may be carried in suspension. A mountain stream in spring is often opaque and milky with suspended sediment.

Another transport method is by wind. Occasionally, very fine dust gets carried high into the atmosphere and is carried great distances. Red dust from the Sahara sometimes travels as far as Britain and northern Europe to coat windows and cars. Wind can also transport particles by saltation along the ground and in suspension as dust.

Figure 2. Oak Ridges Quarry, Ontario. Here you can see layers of gravel that were deposited during glaciation.

Figure 3. Dundas Quarry, Ontario. The sedimentary rock shown here contains various minerals, such as calcite.

Deposition

Weathered rock can be deposited as a sediment by three processes:

Settling out of particles – Fragments of rock carried in suspension drop out of the transporting medium (wind or water) when the speed of travel slows down (Figure 2). The size of particles that settle is controlled by this speed. This process can lead to graded bedding as in greywackes where the largest-sized grains settle out first and are followed by smaller and finer grains. This can be used to tell which way is up in a rock bed that has become tilted.
Chemical precipitation – Minerals, such as calcite and dolomite, may occur as precipitates from fluids in rock pores and also directly from sea water. The 'fur' which you can find in a kitchen kettle is a good example of chemical precipitation. When hard water is boiled in the kettle or a pot, the minerals in the water get deposited on the bottom and sides of the kettle or pot. In the same way, if sea water, is heated, some of the chemicals that were in solution in the water may precipitate out.
Experiment: Set an aluminum pie pan of ordinary tap water on a window sill, balcony or porch and let the water evaporate. See the minerals that are left when the water is gone.
Organic Precipitation – All sea creatures which have hard parts, such as skeletons or shells, get the materials for their bones and shells either from sea water or from eating other organisms. These creatures eventually die and their remains become part of the sediment on the sea floor. Some organisms, such as corals, often build a rigid framework which may be preserved pretty well intact when they get buried.

Coal

Coal is a sedimentary rock, that is not formed from minerals or other rocks. It is a special case because it is formed from vegetable material – trees, ferns, plants of any kind – that has been buried in a swamp or by flooding that buries it in mud. As time passes, the material gets buried deeper. A lot of pressure and heat are put on this material the deeper it is buried. Finally, this material becomes rock formed from the carbon in the vegetable matter. Coal often shows fossil patterns of ferns and leaves from the original plants.

The final sediment may consist of one or more of the above three processes.

Sequoia fossil
Figure 4. Sequoia stem (Giant Redwood) with an alder leaf (Alnus). Cache Creek Formation, British Columbia. Age: Eocene. Photo by Dirk Schmid.

Fossils

Fossils form in sedimentary rock when an animal or plant becomes buried in mud or sand before it is eaten or before it rots. If air is excluded, such as it would be in thick mud, over thousands of years the bones turn to stone by absorbing and exchanging minerals with the surrounding soil and water. This would produce a fossil. Or the plants would leave an impression, an imprint, such as you frequently see in lumps of coal (Figure 4).

From these fossils and impressions in sedimentary rock, we can tell what kind of plants and animals lived at a certain period in the very ancient past.

Why are sedimentary rocks important?

Sedimentary rocks are important economically because they contain useful minerals and provide reservoirs for water, oil, and gas. Their formation is of interest too, because many of the processes by which they are laid down can be seen at the Earth's surface today.

Sedimentary rock is always used to see what happened in the past. Different types of sedimentary rock can tell us different things about what the land and sea were like in the past, thousands and millions and billions of years ago.

Types of sedimentary rock:

Shale is formed from mud. Shale tells us there was a swamp or river bottom, for example.
Sandstone was formed from sand. It could have been a sandbar in a riverbed or sand dunes in the desert.
Coal is formed from dead plants such as trees and ferns, and so we believe there was a forest in that place at one time.
Fossiliferous rock contains fossils.

This article may not be copied, distributed or reprinted in any form without the author's permission. To contact the author, please use the e-mail address provided. If you are unable to contact the author, please contact the Canadian Rockhound. Authorized reprints must acknowledge the author, original source and the Canadian Rockhound, and include the website URL address of the Canadian Rockhound.

Top of Page