conglomerates are coarse grained rocks dominantly composed of gravel sized particles that are typically held together by a finer grained matrix. these rocks are often subdivided into conglomerates and breccias. the major characteristic that divides these two categories is the amount of rounding. the gravel sized particles that make up conglomerates are well rounded while in breccias they are angular. conglomerates are common in stratigraphic successions of most, if not all, ages but only make up one percent or less, by weight, of the total sedimentary rock mass. in terms of origin and depositional mechanisms they are very similar to sandstones. as a result, the two categories often contain the same sedimentary structures.