Consist of more than 30% rounded fragments in a matrix (groundmass) of sand and clay. The fragments or pebbles may be derived from volcanic, sedimentary or metamorphic rocks. Conglomerate is a rock consisting of individual stones that have become cemented together.
Conglomerates are sedimentary rocks consisting of rounded fragments and are thus differentiated from breccias, which consist of angular clasts. Both conglomerates and breccias are characterized by clasts larger than sand (>2 mm).
Paraconglomerates consist of a matrix-supported rock that contains at least 15% sand-sized or smaller grains (<2 mm), the rest being larger grains of varying sizes.
Orthoconglomerates are defined by texture. They are a grain-supported rock that consists primarily of gravel-sized grains (~256 mm), with less than 15% matrix of sand and finer particles.
In rock types such as paraconglomerates and orthoconglomerates, were the matrix to be removed, the rock would collapse. This is because the larger grains are supported by the matrix and, without it, there is nothing to hold the grains together. Therefore, the higher the percentage of matrix, the more unstable the rock.
They differ to breccias in one main way, this is the round edges of the larger sediment/cobbles due to their being deposited in high to very low energy conditions. Possibly from the result of large storm activity when deposition seas allowed sedimentation.