Conglomerate rock is composed of larger gravel particle sizes (larger then sand) with a finer-grained matrix all cemented together.

Conglomerate is made of rounded or semi-rounded rock fragments cemented
together.  The rounding of the fragments implies that the fragments were transported a substantial distance from their source and were abraded in contact with other moving fragments.  The rounded fragments were probably deposited along a stream channel or a shoreline.  Fragments within a conglomerate are pea-sized and larger.  An older name for conglomerate is “pudding stone”.

Conglomerates are classified in terms of both their rounding and sorting.  Clast size can range from Boulder, to Cobble to Pebble to Granule.  They are deposited in a variety of environments:  deep-water marine, shallow marine, fluvial, alluvial, and glacial.

When a series of conglomerates accumulates into an alluvial fan, the resulting rock is called a fanglomerate, which form the basis of large oil fields such as those in the North Sea.

Spectacular examples of conglomerates can be seen at Montserrat, a mountain near Barcelona or near Crestone, CO at the foot of the Sangre de Cristo Range.  Typically, conglomerate is a coarse, irregular and friable material not well suited as a building stone.  However, the Roxbury Conglomerate (Puddingstone) near Boston is well known for its strength.  Puddingstone can be cut into blocks for foundations and irregularly shaped stones can be used for field walls.