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FORMATION OF SEDIMENTARY ROCK

Your teacher will provide samples of several sedimentary rocks, such as conglomerate (say "con-GLOMM-er-itt")(it can also be called "aggregate"), sandstone, shale, and limestone. Each of these rocks was formed underwater by the deposit of different layers of materials brought along by the current of the ocean or a river, or even maybe a stream.

The rocks are of different types because of the varied materials that were deposited over a period of time.

Particles of silt that are carried along by water (or wind), and then settle somewhere are called "sediments." ("SEDD-ii-ments") When they harden, and rocks are formed from their materials, those rocks are called "sedimentary rocks." ("sedd-ii-MENN-tary") (See activity No. 141 06.)

Look carefully at the samples of rocks you are studying today, and make a list of the characteristics of each one. Your geology notebook could have a page for each type of rock, listing the different kinds within the types, along with the other information you have about them and your own observations. You should list their colors, their weight (relative to the size of the individual rock), whether you can see crystals, how easy it is to fracture, and any other stuff you think is important.

Examine broken pieces of sedimentary rock with hand lenses and try to identify the different kinds. To identify the samples, some of the following information might be helpful:

a. Conglomerate (sometimes called "aggregate").

Conglomerate forms underwater and is made up of stones and pebbles that are cemented together.

Students can see these pieces in the rock and can probably break some away from the specimen.

Look closely at a sidewalk (an artificial conglomeration) to see how similar it is in composition to conglomerate.

b. Sandstone.

Sandstone forms underwater and is made up of tiny grains of sand which are cemented together. You can clearly see the sand grains with a hand lens.

By rubbing two pieces of sandstone together over a sheet of black paper, you can collect and study some of the grains of sandstone.

Sandstone may be various shades of gray, yellow, or red. If sandstone is brown or orange, it contains some iron.

c. Shale.

Shale forms underwater and is made up of clay or mud that is cemented together.

By rubbing two pieces of shale together over black paper, you can collect and study some of the tiny shale fragments with a hand lens. If a few drops of water are
placed on the shale, it will give off an odor of mud or clay.

Shale varies greatly in color and may be shades of gray, green, yellow, black, or red.

d. Limestone.

Limestone forms underwater and is made up of shells and skeletons of animals that lived long ago. Often the shells are worn, making them difficult to recognize.

A vinegar test can be used to identify limestone. To do the test, set a piece of limestone in a dish and put two other rocks which are not limestone in two other dishes.
Pour some vinegar on each. You will see bubbles and hear a fizzing sound in the limestone dish.

You will realize that such a reaction takes place only with limestone rocks. To further demonstrate how vinegar can be used as an indicator of limestone, pour some
vinegar on a piece of chalk. Note the reaction. (Chalk is a very fine-grained limestone.)

Samples of natural limestone may be white, gray, or red.

Most caves are "made" of limestone. Many different kinds of mineralized water flows into an area where there is a large limestone deposit. The limestone is easily dissolved by the mineral waters, and pore spaces form in the limestone. The more water that flows through the pore spaces, the larger the pores become, until they are actual caves. The erosion of water flowing through caves causes interesting formations in the walls and ceilings of the caves.

Limestone is fun!

 
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