CONGLOMERATE is a sedimentary rock made up of pebbles or other rock
fragments cemented in a background or matrix of finer material,
generally silica, calcium carbonate, clay, iron oxide, or a mixture of
these substances. The rounded rock fragments have been worn by being
rolled in streams and along beaches.
[Illustration: uncaptioned]
If the pebbles embedded in the matrix are sharp and angular, freshly
broken and not worn, the rock is called breccia and is generally found
near the place where the fragments originated. Conglomerate or breccia
may be made up of any type of rock or mineral, most commonly durable
material such as chert, quartz, quartzite, granite, and gneiss.
In Illinois, conglomerates commonly are found at the base of sandstone
formations and as beds in the lower “Coal Measures.” They also are found
in some gravel deposits.