Geology 101: How Conglomerates Happen

A lot of sedimentary rocks are composed of other sedimentary rocks. But no rock is more obviously composed of other rocks than conglomerate.1

Geology’s technical language is both an annoyance and sometimes lovely. The language can be a barrier to understanding. WC’s geology text described conglomerate as

A coarse-grained clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts, e.g., granules, pebbles, cobbles, and boulders, larger than 2 mm (0.079 in) in diameter. Conglomerates form by the consolidation and lithification of gravel. Conglomerates typically contain finer grained sediment, e.g., either sand, silt, clay or combination of them, called matrix, filling their interstices and are often cemented by calcium carbonate, iron oxide, silica, or hardened clay.

WC will endeavor to translate: “Clasts” are the rocks in the conglomerate. The “matrix” is the finer material surrounding the clasts, that helps hold the conglomerate together. Conglomerate is clasts and matrix that got buried together and, through a variety of processes, got glued together sufficiently to become a rock themselves. Here’s an example of the process happening in the Salmon River Canyon, just upstream from Riggins, Idaho.

Buried and then partially exposed gravel bar, Salmon River Canyon, Idaho

Buried and then partially exposed gravel bar, Salmon River Canyon, Idaho

It’s a buried gravel river bar. River cobbles are the clasts. Sand and silt are the matrix. You can see a landslide topping the gravel bar has started the burial process. The landslide deposited dirt and sharp, angular rocks on top of a gravel bar along the river. A few more landslides and the weight of the overburden, combined with some water percolating through will glue the cobbles and sand into conglomerate. Or the river channel may shift first, transporting the whole mess further downstream, where it will replicate. Geology is messy, and a lot more random than geologists might like.

Geologists distinguish conglomerates from breccia. A breccia is also chunks of rock glued together, but the clasts in a breccia show no or little evidence of erosion, they are not “rounded or subangular,” but instead are still sharp.

Breccia boulder, Seven Devils Formation, Idaho

Breccia boulder, Seven Devils Formation, Idaho

This boulder is about four feet in diameter, and composed of fragments of the volcanic islands that were the Seven Devils Terrane before they met that geologic meat grinder. WC’s mineral skills are weak, but it seemed to be composed of chunks of partially metamorphosed volcanics, limestone and even less recognizable stuff, mashed together into a real mess, broken off by erosion to roll down a steep hillside into the Little Salmon River Canyon. But the key, what makes it breccia and not conglomerate, is those unweathered, angular chunks.

The line between conglomerates and breccias is a lot fuzzier than geologists pretend.

Conglomerates can be created in marine environments as well.

Marine Conglomerate, Kodiak Island, Alaska

Marine Conglomerate, Kodiak Island, Alaska

Here fragments of shell are the clasts embedded in a matrix of fine-grained sediment.

And, pretty amazingly, not all conglomerates are created on Earth.

Conglomerate on Mars, photographed by Curiosity. Photo: NASA/JPL-Caltech/MSSS and PSI

Conglomerate on Mars (left), photographed by Curiosity. Photo: NASA/JPL-Caltech/MSSS and PSI, and on Earth (right)

The Mars Rover Curiosity photographed this conglomerate on the surface of Mars, in an ancient stream bed from a time when Mars had enough running water to weather small rocks and create stream pebbles and cobbles.

Conglomerates aren’t a major part of the geological world. But they are present in mountains and the bones of mountains. Some are being made right now.


  1. Warning to Real Geologists: simplifications ahead. Not oversimplifications, WC hopes, but yeah, simplified. 

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