Conglomerate is a sedimentary rock usually composed of rounded quartz pebbles, cobbles, and boulders surrounded by a matrix of sand and finer material, and cemented with silica, iron oxide, or calcium carbonate. The rock fragments are rounded from being rolled along a stream bed or a beach during transportation. If the fragments embedded in the matrix are angular instead of rounded, the rock is called a breccia (pronounced BRECH-i-a).
The Rockcastle and Corbin Sandstones (Pennsylvanian), which border the Eastern Kentucky Coal Field, contain conglomerates that are very resistant to weathering and cap the tops of mountains throughout much of their extent. Excellent exposures of the Rockcastle occur at Cumberland Falls in Whitley County and along Pine Mountain. The Corbin Sandstone outcrops prominently at Natural Bridge in Powell County. Certain poorly cemented conglomerates may be crushed and used for the same purposes as unconsolidated sands and gravels. Levi Jackson State Park has a collection of millstones that are conglomerates and coarse sandstones that were used for grinding corn and wheat.