In geology, a conglomerate is a rock consisting of other stones that have been cemented together. Conglomerates are sedimentary rocks consisting of subangular clasts and are thus differentiated from breccias, which consist of angular clasts. Both conglomerates and breccias are characterized by clasts larger than sand (>2 mm).

There are two varieties of conglomerate, defined by texture: paraconglomerates and orthoconglomerates.

Paraconglomerates are one of two varieties of conglomerate rock defined by texture consisting of a matrix supported rock that consists of at least 15% sand sized 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) and less than 15% matrix grains (<2 mm, ie. 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.

Fanglomerate

A spectacular example of conglomerate, the Crestone Conglomerate, may be viewed in and near the town of Crestone, Colorado at the foot of the Sangre de Christo Range in Colorado's San Luis Valley. The Crestone Conglomerate is a metamorphic rock statum and consists of tiny to quite large rocks that appear to have been tumbled in an ancient river. Some of the rocks have hues of red and green.

Another example of conglomerate may be seen in the domed hills of Kata Tjuta in Australia's Northern Territory.

When a conglomerate is formed deep within alluvial fans in desert environments, the resulting rock is often called a fanglomerate.