Common as Dirt

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Published on: Feb. 2, 1998

Last revision: Oct. 28, 2010

In fact, it is so much more resistant to breakdown than the surrounding carbonate rocks, that the chert fragments remain long after the limestone or dolomite has weathered away and developed into soil. This explains the abundant chert fragments in, and on top of, many Ozark soils.

Soil scientists classify and describe soils. They have given the name "Clarksville Very Cherty Silt Loam" to one of our common soils formed from cherty limestone. Clarksville soil is deep, but you wouldn't know by looking at it; the surface is mostly covered with rocks. But among the rocks there is some soil. From the surface down to 60 inches, Clarksville soil contains from 40 to 60 percent chert fragments. Profiles of cherty soils, including Clarksville, are exposed in road cuts throughout the Ozarks.

When rain erodes steep, rocky soils, some of the rock moves down the slopes and into streams and valley floors. Other rocks are redeposited within the streams. Finer soil material is carried away, leaving cobble and gravel-sized chert rocks in massive deposits in and along our creeks and rivers.

Those gravel bars that make such good boat landings and camping spots came from the chert that millions of years ago lay trapped in solid beds of limestone or dolomite. It took an unfathomable amount of time for these rocks to become free of the limestone so they could help form a gravel bar. But then, what's time to a rock?

Although chert usually is not thought of as a valuable mineral resource, chert gravel is used for surfacing roads in counties where the gravel is readily available. The chert is mined from deposits along the streams, providing an inexpensive source of road gravel. Chert gravel is also is used in asphalt in the construction industry. It is sold for landscaping as "river gravel." Mining directly from streams harms fish and other stream life, but there are ways to extract gravel from a stream's floodplain with minimal damage.

Chert once occupied a more critical status as a useful commodity: Stone Age people favored it for making tools. Hard, tough, fine-grained and brittle, chert can be fractured and chipped by a skilled craftsman to create intricate shapes and sharp edges. For thousands of years, chert knives, scrapers, spear and arrow points, drills, hoes, axes, grinding tools and ceremonial objects represented the peak of art and technology.

Some kinds of chert made better tools than others. Exposure and

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