Conglomerates and Breccias

A clastic rock made of particles larger than 2 mm in diameter is either a conglomerate or a breccia. A conglomerate has rounded clasts while a breccia has angular clasts. While both can be deposited by water currents or rock falls, breccias also can form as the result of grinding along faults or the collapse of caves in limestone. It takes just a little bit of transport to round the edges off of a clast, so conglomerates are more common.

Rounded gravel and cobbles clearly identify this as a conglomerate. The pipe clearly indicates that this photo was taken in the late 1960s or early 1970s. Photo: Norris W. Jones
The angular gravel and cobbles mark this as a breccia. In this case, the clasts are in fact limestone or dolomite, so this is a carbonate breccia. Photo taken on the surface of an alluvial fan near Resting Springs Pass, CA
The highly angular boulders and cobbles again indicate a breccia. Again, this is a limestone breccia. In this case, the breccia probably formed along a fault or as the result of a cave roof falling in. Death Valley, CA
This is another shot along the same canyon wall as shown above. It is a spectacular breccia. Some of these clasts are HUGE. Death Valley, CA
This one is trickier. The clasts are clearly gravel to cobble sized, but are they angular or rounded? If you zoom in on the image, you will see that the edges of the clasts are rounded, so this counts as a conglomerate. West of Pittsburgh, PA

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