Sand is a loose, uncemented sedimentary deposit made up of fragments of
weathered rocks and minerals. These fragments must be of a certain size
(between ¹/₁₆ millimeter and 2 millimeters in diameter) in order to be
called sand grains. The largest sand grains are about the size of a
pinhead. Sand grains are smaller than the fragments known as _granules_;
they are larger than those known as _silt_.
Many sands are made up chiefly of grains of quartz. This mineral is
plentiful and does not easily weather away. In addition, rock fragments
and many other minerals, such as feldspar, mica, gypsum, magnetite, and
garnet, are found as sand grains.
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.]
Ordinary sandstones are seen at the surface in many localities in Texas,
and a number of them have been used as building stones. Some of the
places where sandstones occur are in the Cambrian and Pennsylvanian
formations of the Llano uplift area of central Texas and in the
Pennsylvanian, Permian, and Lower Cretaceous formations of north-central
Texas. Tertiary sandstones occur in the Texas Gulf Coastal Plain, and
Triassic sandstones are found along the edges of the Texas High Plains.
Sandstone is also found in many formations of the Trans-Pecos country of
west Texas.
Sandstone. _See_ Sand and Sandstone.
Satin Spar. _See_ Gypsum.