Rains wash many of the sand grains and other weathered rock and mineral fragments into creeks and rivers. These streams may carry the sand and other sediments long distances before depositing them. Today, we find sands along the banks of many creeks and rivers in Texas and along the beaches of the Gulf of Mexico. The sand in the rivers is in transit to the Gulf. In addition, sand occurs at the surface in other Cenozoic formations and in some of the Paleozoic and Mesozoic formations of Texas. Sand has many uses. Much _building sand_, which is used in mortar and concrete, is produced from numerous sand and gravel pits in Texas. Pure quartz sand that can be used to make glass is known as _glass sand_. Some of it is found in north-central Texas in Lower Cretaceous formations. A large glass sand quarry is located at Santa Anna in Coleman County. Along the Gulf Coastal Plain, sand that is used in glassmaking occurs in Eocene Tertiary strata. A coarse-grained sand, _blast sand_, is used with compressed air to clean the walls of brick and stone buildings and to carve designs on monument stones, such as marbles. Some coarse sand is also used as a _filtering_ sand in purifying water. These types of sand have been produced from the Gulf Coastal Plain as well as from other areas of Texas. Sand grains, when nature cements them together, make up the sedimentary rock _sandstone_. Some sandstones form when underground water carrying dissolved mineral matter moves through loose sand. As the dissolved mineral matter comes out of solution, it forms a cement that binds the sand grains together. The cement may be material such as calcite (calcium carbonate), quartz, chalcedony, or opal, which are silica minerals, and limonite and hematite, which are iron oxides. Clay also may serve as a cement. It is either deposited along with the sand or is formed from weathered feldspar sand grains. The color of the cementing material helps determine the color of the rock. Iron oxide cement, for example, causes the sandstone to have a reddish, yellowish, or brownish color. Sandstones also are white, black, gray, green, or cream colored. Ordinarily, sandstones break through the cementing material, not through the sand grains. Thus, the broken surface of the rock feels rough and gritty. Some quartz sand grains, however, are tightly cemented with silica to form an extremely hard and compact rock. If this rock breaks smoothly through the grains instead of between them, it is known as _quartzite_. Some of this sedimentary quartzite occurs in the Texas Gulf Coastal Plain in the Tertiary Catahoula strata. (Another kind of quartzite is described on pp. 84-85.) [Illustration: Sandstone from the Eocene Wilcox Group of strata of northwestern Zavala County, Texas.]