The cooling and hardening of hot, molten magma also takes place below the earth’s surface. Here, the magma cools slowly to form rocks made up of mineral grains that are large enough to be readily visible. These rocks are known as _intrusive_ igneous rocks. We know that they are present below the surface in Texas because of wells drilled in many areas of the State. In Pecos County, a well reached granite, an intrusive igneous rock, at a depth of 16,510 feet. Other wells in Texas have reached the granite basement rocks at much shallower depths. But not all intrusive igneous rocks in Texas are found underground. In the Trans-Pecos country of west Texas, in the Balcones fault zone, and in the Llano uplift of central Texas, some are now seen at the surface. They, like all intrusive rocks, were formed below the ground, but earth’s processes of uplift and erosion have gradually uncovered them. [Illustration: Intrusive igneous rocks form from molten rock material (magma) that cools and hardens beneath the earth’s surface.] SEDIMENTARY ROCKS Sedimentary rocks are made up of sediments, which are rock and mineral grains that have come from weathered rocks of all kinds. Rocks are weathered when water, ice, snow, wind, and other agents cause them either to dissolve, as table salt does when put in water, or to break apart, as old pavement commonly does. Soils Some of the broken-down rocks, along with associated plant and animal matter, develop into soils. When you examine soil with a magnifying glass, you may be able to see some of the small rock and mineral grains that still remain in it. Some soils have formed on top of the rocks from which they came, and some have been moved in from another place. [Illustration: Soils develop from weathered rock and associated organic material.] SOIL SUBSOIL WEATHERED ROCK BEDROCK Sedimentary Rock Materials in Broken Fragments Water and wind not only weather the rocks and soils but also move the weathered materials (the sediments) and deposit them in other places. Whenever you see a dust or sand storm, or a muddy creek or river, you are observing the movement of sediments by wind and water to other land areas or to the sea. The combination of weathering and movement is called _erosion_. [Illustration: Conglomerate from Webb County, Texas, is composed of rounded gravel that has been cemented together.] Some of the rock fragments carried by water are still fairly large when they reach their destinations. On the basis of size, they are called _boulders_, _cobbles_, _pebbles_, and _granules_. Loose deposits of these larger-size sediments make up what is known as _gravel_. Nature cements gravels together to form rocks such as _conglomerates_ (made up of rounded gravel) and _breccias_ (made up of sharp-cornered gravel).