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Sedimentary Rock
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Sedimentary rocks are the 'used cars' of the rock
world. They are formed at or near the Earth's surface out of bits and pieces of
other rocks, animals, plants, and from the crystallization of minerals. The
components that make up the rock ultimately lead to the name of the rock. There
are three classes of sedimentary rock: clastic, chemical and biologic. |
Clastic Sedimentary Rock
The most common sedimentary rocks are clastic - they are formed from bits and
pieces of pre-existing rock which have been weathered, eroded and transported to
an area where they are deposited together (usually in a low place, like in a
valley, an ocean, a lake or other low spot on the Earth). Nature hates ups and
downs; it is erosion that tears down mountains and fills the valleys in an
attempt to have a flat planet.
For clastic
rocks, the size of the individual grains that make up the rock determine what
type it is. Let's start with the bigger grain-sized rocks first and work our way
down.
| Conglomerate - This rock type can contain very large, even
boulder size particles (called clasts), along with very small sand grained
size particles and everything in-between. This rock forms in a river or
stream, or any place where flooding takes place. This is an example of a
poorly sorted rock - the clasts that make up this rock are all different
sizes. We know this rock was deposited by water, notice the round shape to
all the clasts. |
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| Breccia - This rock type is the same as conglomerate, except
that the clasts are not rounded, but are jagged or angular. This rock is
deposited in a non-water environment, or in an area where water did flow
long enough to round the edges of the clasts. |
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| Sandstone - This very common rock is made of sand-sized
grains cemented together. This rock feels like sand paper. The rock is
made of uniformly sized particles - this is called a well-sorted rock. The
rock on the right also exhibits "cross-bedding". This occurs
when wind or water direction changes during deposition. The depositional
environment of this rock includes sand dunes, beaches, and near-shore
ocean environments. |
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| Siltstone - This rock is also very common. It is composed of
silt-sized particles, just large enough to detect with the naked eye. The
rock is well sorted, and sometimes forms fine layers. This rock is
commonly found with sandstone, indicating similar depositional
environments. Siltstones are most commonly formed a little farther
offshore than sandstone in deeper water of a lake or ocean. |
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| Mudstone - This rock is made of very fine grains, too small
to see with the naked eye. It is formed farther offshore than silstone. |
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| Shale - This rock is composed of extremely fine grained
particles that are compressed together to form thin layers like in the
photo at right. Shale is often dark in color due to the lack of oxygen
from the tight fit of the individual grains. It is commonly very soft and
easy to break apart. |
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Let's keep going....
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