A conglomerate (IPA: ) is a rock consisting of individual stones that have become cemented together. Conglomerates are sedimentary rocks consisting of rounded fragements 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.



A spectacular example of conglomerate, the Crestone Conglomerate may be viewed in and near the town of Crestone, at the foot of the Sangre de Cristo Range in Colorado's San Luis Valley. The Crestone Conglomerate is a metamorphic rock stratum 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.