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Sedimentary
Sedimentary rocks are formed from parts of the Earth, such as sand, shells and pebbles. These rocks are often identified by layers; when broken open, you can see the lines on the rock which represent different periods of time. These lines are formed when one element is pressed onto an existing rock element to form a new layer. Sedimentary rocks are often formed from sediment that is carried down streams and pounded into rocks by water.
Metamorphic
The word "metamorphic" comes from the Greek word for "change" and "form." Metamorphic rocks are rocks that have changed after millions of years of pressure. Unlike sedimentary rocks, metamorphic rocks are formed from other rocks that have been subjected to heat and pressure over time. These rocks are especially abundant near volcanoes because the heat helps the rock change shape and texture. The grains in metamorphic rocks are larger than those found in sedimentary rocks.
Igneous
Igneous rocks are formed from cooled lava. After lava spews out of a volcano and flows onto the ground, it hardens and becomes igneous. This is the oldest of all rock types. In fact, the base of continents are formed from igneous rocks. The Earth was once covered entirely by water, but lava eventually spewed out of the Earth's core to form land. Igneous comes from the Greek work meaning "fire."
Glacial Till
All three types of rocks can be found in glacial till, which are deposits left by massive glaciers during the ice age. As the glaciers extended, they shifted through the continents, carved up pieces of the Earth and carried them long distances. As the glaciers melted, the glacial till was deposited in piles called moraines. Some large glacial till deposits contain diamonds and are the location of mining operations. Glacial till is often dense and gray.