clastic Rock Types

Clastic Rock Types
Clastic rocks are grouped by the dominant grain size and the types of materials. There are 4 main size groups each with subgroups by types of materials. The groups are:

Conglomerate - largest grains are pebbles to boulders with smaller grains mixed in. If the peices are angular rather than rounded they are called Breccias. Subgroups are named by the major type of large pieces or by the type of cement holding the rock together.

Sandstone - dominant grains are sand sized. named for the major grain size present (fine, medium, course, etc).

Arkose - contains Quartz and >25% Feldspar grains

Greywacke - contains Quartz and >25% rock fragments

Siltstone - dominant grains are siltsized.

Shale - dominant grains are clay sized. Named for cements or included materials or for color.

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Website created: January 22, 1998 Website last updated: November 18, 2000