Engineering geology

Conglomerate and Breccia

Conglomerate consists largely of rounded gravel-size particles (Figure 1). As Figure 2 indicates, these particles can range in size from large boulders to particles as small as peas.

Conglomerate and Breccia
Figure 1 – Conglomerate The gravel size particles in this rock are rounded. (Photo by E. J. Tarbuck)


Conglomerate and Breccia
Figure 2 – Particle size categories Particle size is the primary basis for distinguishing among various detrital sedimentary rocks. (Breccia photo by E. J. Tarbuck; all other photos by Dennis Tasa)

The particles are often large enough to be identified as distinctive rock types; thus, they can be valuable in identifying the source areas of sediments. More often than not, conglomerates are poorly sorted because the openings between the large gravel particles contain sand or mud (Figure 3).

Figure 3 – Poorly sorted sediments – Gravel deposits along Carbon Creek in Grand Canyon National Park are poorly sorted. (Photo by Michael Collier)

Gravels accumulate in a variety of environments and usually indicate the existence of steep slopes or very turbulent currents. The coarse particles in a conglomerate may reflect the action of energetic mountain streams or result from strong wave activity along a rapidly eroding coast. Some glacial and landslide deposits also contain plentiful gravel. If the large particles are angular rather than rounded, the rock is called breccia (Figure 4).

Breccia
Figure 4 – Breccia – The gravel-size sediments in this rock are sharp and angular. (Photo by E. J. Tarbuck)

Because large particles abrade and become rounded very rapidly during transport, the pebbles and cobbles in a breccia indicate that they did not travel far from their source area before they were deposited. Thus, as with many other sedimentary rocks, conglomerates and breccias contain clues to their history. Their particle sizes reveal the strength of the currents that transported them, whereas the degree of rounding indicates how far the particles travelled. The fragments within a sample identify the source rocks that supplied them.

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