This lab is intended to introduce you to sedimentary rocks, especially those types that you are likely to see right here in Ohio. Sedimentary rocks are important in historical geology for several reasons:

1. they make up 2/3 of all the rocks exposed at the Earth's surface,
2. they contain most of the fossil record
3. they preserve structures which serve as clues to ancient depositional environments.

You will be investigating sedimentary structures and fossils in future labs. After you have completed this activity, you will be able to identify several of the basic types of sedimentary rocks. You will also construct a rudimentary geologic map of the state of Ohio.

Igneous rocks (granite, basalt, etc.) are formed when molten rock material, magma or lava, cools and solidifies. Metamorphic rocks (gneiss, quartzite, etc.) are formed when heat and pressure change some of the characteristics of existing igneous or sedimentary rocks. Sedimentary rocks are formed as result of the deposition and lithification of sediments. There are numerous ways to classify sedimentary rocks (over 50 classification schemes have been proposed for sandstone alone!), each has its strengths and weaknesses. You will use a simple key to identify some of the basic types of sedimentary rocks based on easily observable characteristics.

There are three general categories of sedimentary rocks: clastic, nonclastic, and biogenic. The deposition and lithification of fragments of preexisting rocks (sedimentary, igneous, and/or metamorphic) form clastic rocks. Nonclastic rocks are mainly formed by chemical or biochemical precipitation. Rocks classified as biogenic mostly consist of plant and animal remains.

A geologic map shows the distribution of rocks at the Earth's surface. In many places, unconsolidated materials like soil and glacial till obscure rocks outcrops. A geologic map shows the location and type rocks are found beneath all that loose stuff. Geologists often have to base their maps on limited data, inferring what rocks exist in covered areas between rock outcrops. When constructing your geologic map, you will have to do the same, inferring rock systems and geologic contacts between systems. In the first lab, you learned that geologic time is divided into eons, eras, and periods. A package of rock units deposited during a particular period of geologic time is known as a system. For example, the rocks deposited during the Mississippian Period are known as the Mississippian System.

For more information visit the Ohio Department of Natural Resources Geologic Survey (ODNR-GS) Site.