Ask-A-Geologist Question #9: What grains are in conglomerate rocks?

Hi everyone! We have another great Ask-a-Geologist question from one of our readers. Today’s question is about conglomerate rocks and exactly what types of grains form these special samples.

What grains are in conglomerate rocks?

Hi Matthew, sedimentary conglomerates are made up of many different types of grains from sand to various types of igneous, metamorphic, or sedimentary rocks. Conglomerates typically have rounded grains that were weathered over time by wind or water in a river, for example. Unlike some rocks which have a specific mineral makeup, conglomerates can have many different grains of rocks and minerals.

Conglomerate Sedimentary Rock

Now, this sample is a conglomerate and this one, which looks very similar, is a sedimentary breccia. These two rocks are very similar except that the conglomerate has rounded grains and the breccia has angular grains.

Features of Sedimentary Conglomerate Rocks

Color:  Varies based on what minerals are present.
How it Forms:  Conglomerate is formed in an ocean or river setting when pebbles are glued together with smaller particles of sand and clay.
Minerals:  Can contain any type of rock fragments.
Texture:  Very Coarse-grained pebbles with fine-grained silt and sand.
Uses:  Building materials.
Features:  Can be confused with breccia. However, breccia has angular fragments and conglomerate has rounded fragments.

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