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Rock Descriptions Acitivity

Conglomerate 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.

Sandstone 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.

Gneiss  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.

Obsidian 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.

Marble 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 calcite.

Granite 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, igneous rock


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