File:Conglomerate (Tokio Formation, Upper Cretaceous, ~85 Ma; Crater of Diamonds State Park, Arkansas, USA) 3.jpg

English: Conglomerate (lithic conglomerate) from the Cretaceous of Arkansas, USA. (8.0 centimeters across at its widest)

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.

Conglomerate is a siliciclastic sedimentary rock consisting of a mix of large & 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.

Seen here is a lithic conglomerate that is dominated by subrounded to subangular lithic pebbles and sand. This is from a lithified gravel unit that caps a famous Cretaceous-aged diamondiferous lamproite diatreme in Arkansas - the Prairie Creek Lamproite.

Stratigraphy: Tokio Formation, Upper Cretaceous, ~85 Ma

Locality: Crater of Diamonds State Park, southern Pike County, southwestern Arkansas, USA