File:Travertine-cemented conglomerate 4.jpg

English: Sedimentary rocks form by the solidification of loose sediments. Loose sediments become hard rocks by the processes of deposition, burial, compaction, dewatering, and cementation.

There are three categories of sedimentary rocks: 1) Siliciclastic sedimentary rocks form by the solidification of sediments produced by weathering & erosion of any previously existing rocks. 2) Biogenic sedimentary rocks form by the solidification of sediments that were once-living organisms (plants, animals, micro-organisms). 3) Chemical sedimentary rocks form by the solidification of sediments formed by inorganic chemical reactions.

Most sedimentary rocks have a clastic texture, but some are crystalline.

Seen here is conglomerate, a siliciclastic sedimentary rock consisting of a mix of large and small grains - it is poorly sorted. The larger grains (pebbles or cobbles or boulders) in conglomerates are rounded to subrounded in shape. The finer-grained matrix is usually sand or mud. Most conglomerates were deposited in stream/river environments or alluvial fan environments or some very shallow marine environments.

This specimen is unusual in that the pebbles have been cemented together by travertine - each clast is coated with a thin travertine crust. There is still significant porosity (empty space) between the pebbles. The intragranular areas lack sand or mud. In terms of the rock's origin, it's possible that hot spring water rich in dissolved calcium carbonate moved through a pebble deposit.

Provenance: unknown

Locality: unrecorded, but collected out-of-place in the western USA