Conglomerate rocks are formed by the sedimentary rock process, which is: erosion, transport, deposition and cementation. Two characteristic properties of conglomerate rock are that the sedimentary particles, or clasts, are greater than 2 millimeters in size, and the clasts are rounded in appearance. Sedimentary rocks with angular clasts are distinguished from conglomerates and are called breccias.
Conglomerates, like other sedimentary rocks, are formed from the detritus of source rocks, which results from weathering and erosion. As the pieces of source rock fall away, they are transported to other locations where they are deposited in layers. In the case of conglomerates, the transport by water causes the clasts to assume their rounded shape. As the sedimentary layers pile up in the stage of the formation process called deposition, pressure compacts the lower layers.
The final stage of the process is cementation. This refers to the development of new minerals between the layered and compacted sedimentary particles. These new mineral growths bind the particles together. The process is helped along by water entering through the pores in the sedimentary layers, and depositing minerals such as calcium carbonate or silica, between the particles. The water-borne precipitates act like cement and also decrease the porosity, or the degree of open space, between the sedimentary particles until new hardened rock is formed.