The Rock - Sandstone
- Sedimentary Rock Type:
- Related to: Shale, conglomerate, siltstone,breccias
- Color: highly variable
- Texture: Sand sized grains
- Origins: River, ocean, glacier and desert deposits
- Common Minerals: Quartz, feldspars,
- Uses: Building material, decorative stones, tiles,
tombstones, monuments, roads, ore of silica for glass, abrasives, aquifers,
Sandstone is a very common
sedimentary rock and perhaps the best
known sedimentary rock. It is formed in many environments. Just
about anywhere there is water, whether frozen or not, in a river or ocean,
there is a chance to form sandstone. Even where there is no water
as in a desert, there is sandstone formation under foot.
As the name implies, sandstone is composed of sand. That seems easy
enough. However there are a few other things to consider. What
is sand? Sand is characterized by any grain that is 0.1 mm to 2.0 mm
in size. Any smaller grain size and you have a shale or
any larger and you have either a conglomerate or a
breccia. The grains
can be composed of individual crystals of various minerals such as
or even be a sand-sized fragment of another rock such as a granite or slate.
A magnifying glass is usually sufficient to distinguish the general
composition of a sandstone. How a sandstone formed is usually important
to geologists and the minerals or rocks that are in it are critical to determine
where the source of a sandstone's composition came from. The roundness
of the grains is also important in determining the amount of distance the
sand has been tumbled before deposition or the closeness of the source to
the final deposit.The lack of fine grains and mud in a sandstone indicates a
relatively high energy environment of deposition such as the wave action on a
beach, the wind sweeping across a sand dune field or the rush of a river
The grains are important to geologists and so are the minerals that cement
them together. Sandstone cements can influence the durability, color,
porosity and usefulness of the stone. Normal cementing agents include
and gypsum. Silica cemented sandstone
is very durable and hard. Calcite cemented sandstone is subject to
acidic dissolution and is more easily eroded. Clay and gypsum cements,
which are soft minerals, tend to produce much softer sandstone and the sand
can sometimes be rubbed off in a person's hands.
The cementing agents also affect the porosity of a sandstone. When
the initial sand is first deposited there are lots of open spaces or pores.
Water, for instance, flows right through sand due to all the pores. But
as the sand turns into rock, the cement can fill in these pores making the
sandstone less porous and less able to allow water to move through the rock.
Generally sandstone is a very porous rock as rocks go and will
have substantial pore spaces. Sandstone is the ideal rock for ground
water and will house substantial aquifers. Petroleum also is a fluid
that flows through sandstone and sandstone is also the best oil reservoir.
Iron oxides, manganese oxides and other impurities can cause bright
and contrasting colors in sandstones. These colors are what gives sandstone
its unique character and ornamental desirability. The colors range
from bright whites, reds, yellows, oranges and even purples and greens. Colored
sandstone is usually intricately banded in multiple colors which enhance
its aesthetic appeal.
Sandstone's banding is due to layers of sand that are deposited with differing
characteristics. Sandstone is formed in many deposits that are episodic
in nature and the resulting layers can be very different from previous layers.
Sometimes the sand is courser or finer than the previous layer and
this difference causes the banding. Dunes and ripples of sand can cut
across earlier deposits and cut off those earlier bands with bands of their new.
This results in a texture call cross-bedding (see picture above) and can be
very intricate and interesting. "Picture Rock" is a type of ornamental
sandstone that can display scenic, almost picture like banding.
Sandstone deposits can be a beautiful part of the natural environment and
apart of the breath-taking views at many natural parks and preserves. The
sandstone deposits of the the Grand Canyon, Zion, Bryce, Mesa Verde, Arches,
and Red Rock National Parks to name a few in just the USA provide millions
of visitors spectacular views.
Mined sandstone can be cut, polished and carved for many uses. It is
used as ornamental rocks for buildings, monuments, grave stones, bookends,
beverage coasters, tiles and many other possible uses. Sandstone can
also provide silica for glass production.
Some mineral ores are found in sandstone. Uranium can concentrate in
uranium minerals in sandstone deposits and many uranium ore bodies are the
result. Heavy minerals such as
diamonds and others can
be found in sandstones from prehistoric placer deposits that are now worked
for their economic value.