conglomerate - display specimen of the Shinarump Conglomerate, Triassic, Hurricane Mesa, Utah

This Early Late Triassic conglomerate is a stream-channel deposit. The yellow brown stain is from oxidation of iron contributed by ash from volcanos associated with subduction at the western edge of the Triassic continent. The Shinarump, resistent to weathering, is often a mesa caprock in southwestern Utah. This was collected from the cap of Hurricane Mesa, in Washington County, Utah.

The stream channels often captured logs when the water level dropped. These became buried and replaced by silica, forming the petrified wood that is common in the Shinarump, a member of the Triassic Chinle Formation. This example looks just like a stream bed deposit and its origin should be clear to students.

A conglomerate is the largest-grained sedimentary rock in the series from mudstone, shale, siltstone, sandstone and conglomerate. The individual pebbles are called clasts, from the Greek word klastos, for broken. As these tumbled downstream they became rounded and less angular. Eventually, far from the source, these clasts would have been reduced to rounded sand grains if they had not been stranded in a channel deposit and lithified.

The pencil is 5.5" long, for scale.

Field photos show the Shinarump cap on Hurricane Mesa with redbeds of the Moenkopi Formation below, the resistant vertical wall of the Shinarump mesa cap, and why conglomerates are tricky to collect - they simply fall apart. In arid areas, sandstone, conglomerate and limestone are cliff formers.

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