Rock Pictures and Descriptions

Little Bits Rockin’ Rock Hunters Rock Descriptions
Sandstone
Sandstone doesn’t signify any particular mineral, but in practice,
sandstone is usually almost all quartz. It is created when grains of sand
are weathered, which means blown around by the wind, or pushed
around by rain, water, or erosion. When it settles down in one place, it
forms layers that are stuck together with the minerals that fall between
the grains of sand. Sandstone is sometimes found where there used to be
small sea areas. It is also formed in deserts or dry places like the Sahara
Desert in Africa. It can also be found around lakes, rivers, deltas, and
shores all over the world. Sandstone forms where sand is laid down and buried. Usually this happens
offshore from river deltas, but desert dunes and beaches can leave sandstone beds in the geologic
record too. Sandstone does not usually contain good fossils because the energetic environments
where sand beds form don’t favor their preservation.
Coal
Coal is called a fossil fuel because it was formed from the remains
of vegetation that grew as long as 400 million years ago. It is often
referred to as “buried sunshine,” because the plants which formed
coal captured energy from the sun through photosynthesis to create
the compounds that make up plant tissues. The most important
element in the plant material is carbon, which gives coal most of its
energy. Source: www.ket.org
Arkose
Arkose is a raw, coarse-grained sandstone, deposited very near its
source, that consists of quartz and a significant proportion of feldspar.
Arkose is known to be young because of its content of feldspar, a
mineral that usually degrades quickly into clay. Its mineral grains are
generally angular rather than smooth and rounded, another sign that
they were transported only a short distance from their origin. Arkose
usually has a reddish color from feldspar, clay and iron oxides—
ingredients that are uncommon in ordinary sandstone.
Source: www.geology.about.com
Shale
Shale, a sedimentary rock formed by the consolidation of mud
or clay, has the property of splitting into thin layers parallel to
its bedding planes. Shale tends to be fissile, i.e., it tends to split
along planar surfaces between the layers of stratified rock. Shales
comprise an estimated 55% of all sedimentary rocks
Limestone
Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of
the mineral calcite. It most commonly forms in clear, warm, shallow marine waters. It is usually an
organic sedimentary rock that forms from the accumulation of shell,
coral, algal and fecal debris. It can also be a chemical sedimentary
rock formed by the precipitation of calcium carbonate from lake or
ocean water.
Most limestones form in shallow, calm, warm marine waters. That
type of environment is where organisms capable of forming calcium
carbonate shells and skeletons can easily extract the needed
ingredients from ocean water. When these animals die their shell
and skeletal debris accumulate as a sediment that might be lithified into limestone. Their waste
products can also contribute to the sediment mass. Limestones formed from this type of sediment are
biological sedimentary rocks. Their biological origin is often revealed in the rock by the presence of
fossils.
Source: www.geology.about.com
Conglomerate
Conglomerate is a clastic sedimentary rock that contains large
(greater than two millimeters in diameter) rounded clasts. The space
between the clasts is generally filled with smaller particles and/
or a chemical cement that binds the rock together. Conglomerate
forms where a sediment of rounded clasts at least two millimeters in
diameter accumulates. It takes a strong water current to transport
and shape particles this large. So the environment of deposition might
be along a swiftly flowing stream or a beach with strong waves. There
must also be a source of large-size sediment particles somewhere up
current. The rounded shape of the clasts reveal that they were tumbled by running water or moving
waves. Source: www.geology.about.com