consists mostly of gravel. The particles may vary largely in size
within any one sample. This variety of size is called poor sorting where
the rock may consist of rocks as large as boulders as well as pieces
as small as a single pea.
The particles are usually large enough
that it is easy to identify the individual rock types. This information
may be very helpful in identifying the source area of the conglomerate.
Conglomerates usually have large openings between the gravel particles
which is filled by some groundmass such as sand or mud.
is any rock with predominantly sand-sized grains. After shale,
sandstone is the next most abundant sedimentary rock. Sandstone
forms in a variety of environments and often contains important clues
about the origin. These "clues" are derived from the sorting, particle
shape and composition of the sandstone.
Sandstone is generally composed
of uniform grains of quartz cemented together by silica or lime. Most
sandstones were formed under oceans, usually near shorelines, in rivers
or desert areas. It is generally very abrasive and soft. Smaller samples
can often be broken by hand.
is a medium to coarse grained rock which is composed of feldspar, mica,
and quartz. Layers of dark and light rock are usually found in an alternating
pattern. All gneisses have undergone extremely
high pressures and may show evidence
of partial melting which explains the often occurring swirling patterns.
is often called volcanic glass. It forms when lava cools quickly,
as often happens when the flow of lava reaches a body of water. Samples
are usually black, but there may be red or brown streaks. It has
a glossy luster.
is composed chiefly of the mineral calcite and is formed from metamorphosed
limestone. Marble has a wide range of colours, from white to red,
green, black, or in combination. Marble is a non-foliated, coarse,
crystalline rock. When marble is examined by hand it closely resembles
crystalline limestone. Pure marble is white composed primarily of mineral
is usually light-coloured, and contains fine crystals of uniform shape
which is a characteristic of very slow cooling rocks. Granite is generally
composed of a combination of feldspar and quartz, and usually contains
either mica or hornblende. Granite is a coarse-grained, intrusive,