Sedimentary Rocks

Fascinating Fact Seventy percent of all the rocks on earth are sedimentary rocks.

The earth's surface is constantly being eroded. This means that rocks are broken up into smaller pieces by weathering agents such as wind, water, and ice. These small pieces of rock turn into pebbles, gravel, sand, and clay. They tumble down rivers and streams. These pieces settle in a new place and begin to pile up and the sediments form flat layers. Over a long period of time, the pieces become pressed together and form solid rock called sedimentary rock. Most sedimentary rocks form under water. Most of the earth has been covered by water some time in the past. 70% of the earth is covered by water now. So sedimentary rocks are common all over the world. Sedimentary rocks are often rich in fossils.

Sediments can harden into sedimentary rock in two ways.

pressure-As layer after layer of sediments are deposited, the lower layers are pressed together tightly under the weight of the layers above.
cementing-Some sediments are glued together by minerals dissolved in water.

Some examples of sedimentary rocks are sandstone, limestone, conglomerate, and shale. Sandstone is formed from grains of sand pressed tightly together. Sandstones are very common rocks. They are formed from the sand on beaches, in riverbeds, and sand dunes. Sandstones are usually made of the mineral quartz. Limestone is formed from tiny pieces of shells of dead sea animals that have been cemented together. Conglomerate contains sand and rounded pebbles that have also been cemented together. Shale is formed from mud or clay that has been pressed together. Shale forms in quiet waters such as swamps and bogs. Sedimentary rocks are easy to identify because you can actually see the layers.

Below is a summary of the major characteristics of sedimentary rocks.

Classified by texture and composition
Often contains fossils
May react with acid
Often has layers, flat or curved
Usually composed of pieces cemented or pressed together
Has great color variety
Particle size may be the same or vary
Usually has pores between pieces
May have cross-bedding, mud cracks, worm burrows, raindrop impressions

Here's a checklist of some common sedimentary rocks and their characteristics. Look for these same characteristics in the rocks you find, and you'll be well on your way to becoming a rockhound.

Some Common Sedimentary Rocks
Name	Image	Color	Composition
Sandstone		Red or Gray	Sand grains cemented together
Limestone		White to Gray	Calcite and sometimes fossils
Shale		Dark Gray	Compacted mud
Conglomerate		Different Colors	Rounded cobbles and pebbles cemented together

To learn more about rocks and minerals, click next below:

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Copyright March, 2000