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Common as Dirt

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Published on: Feb. 2, 1998

Last revision: Oct. 28, 2010

Some kinds of rocks are hard to get to know personally. Not so with chert. If your tire has ever been punctured by a chert sliver on an Ozark road, or if your air mattress has ever gone flat while camping on a gravel bar, you know what I mean.

Chert, also known as flint, is one of the most abundant rocks in Missouri, and is composed of one of the most common minerals on earth. In fact, in much of Missouri you could say that chert is "common as dirt."

Chert is a hard, fine-grained rock made up mostly of the mineral silica (SiO2). Silica itself contains the elements silicon and oxygen, two of the most abundant elements on earth. Silica is also the main ingredient in glass, obsidian, quartz, many kinds of sand and computer chips. It is a lesser component of numerous other rocks and minerals. Chert and flint are generic names for this siliceous rock, which comes in many colors and patterns.

Missouri chert is usually white, gray or tan, but almost any color is possible, depending on chemical variations in the stone and the details of its formation. Certain colors have their own special names. Jasper, for example, is reddish, orange or yellow chert. Agate is banded in many colors and is sometimes translucent, making it a favorite for jewelry and crafts. The Missouri General Assembly chose mozarkite, an attractive chert with a banded pattern of red, pink and gray, to be our state rock.

Most of Missouri is underlain by carbonate rocks-limestones and dolomites-that formed from sediments on the bottoms of ancient seas. This is the thick, layered rock we see along road cuts and river bluffs. Chert is associated with almost every type of limestone and dolomite in Missouri. It appears as globular-shaped "nodules" and "beads," and in pockets and veins of various sizes and shapes interbedded with the limestone.

Geologists are unsure exactly how the chert formed in the limestone. Two possibilities are that oceans deposited silica along with the calcium carbonate shells of marine life, and the chert formed into sedimentary rock along with limestone. Another theory holds that chert formed later, when silica-rich ground water permeated spaces in the limestone. Whatever its origin, chert exists in limestone, and chert nodules, veins and layers are easy to see in road cuts through chert-bearing carbonate rocks.

Chert is glasslike in its hardness and in its ability to withstand weathering. 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 weathering changes the rock's appearance and tooling qualities. The chert rocks strewn over the ground and piled along streams are inferior for tools. Prehistoric Missourians dug open mines to secure the best chert for making tools. They also traded for special quality stone and stone tools, sometimes over long distances.

Today we find chert flakes in plowed fields that used to be Indian campsites. The abundant flakes are the non-biodegradable litter of thousands of years of tool makers. Occasionally, we may find pieces of the tools themselves and, more rarely, a perfect blade or point, hundreds or thousands of years old. Chert was also important for its ability to produce sparks which could start fires and later, to ignite gunpowder in flint-lock firearms.

In Missouri, chert usually appears in small fragments or narrow bands embedded in limestone and dolomite, or as loose rocks in the soil or along streams. But chert also can be found in sedimentary formations tens to hundreds of feet thick. These solid chert formations resulted from long accumulation of tiny marine animals and sponges that secrete glasslike particles of silica. A good example of this is the almost continuous layers of dense chert within the dolomite bluffs along some Ozark streams.

Some of these "chert reefs" are so persistant over wide areas that geologists use them as reference layers or "marker beds" to help them map the geological layers above and below the reefs. Primitive animals that once lived in these reefs sometimes show up in the rock as chert fossils, preserving the form of the once-living organisms. Even thicker beds of chert form the novaculite deposits in Arkansas' Ouachita Mountains. This special chert is mined to produce the famous "Arkansas stone" whetstones, which sharpen tools all over the world.

Missouri's most impressive beds of thick, solid chert are in the exposed cliffs and glades near Joplin and along Shoal Creek in Jasper and Newton counties. These unusual features are easy to see in Joplin's Wildcat Park and adjoining Wildcat Glade Natural Area on the city's southwest side. Wildcat Glade sports an interesting assemblage of plants-a mosaic of lichen-covered rock, patches of gnarled, stunted oaks and showy wildflowers, such as coreopsis, glade onion, rock pink, prickly pear and Barbara's buttons.

Grand Falls on Shoal Creek is only minutes from Wildcat Park and is perhaps Missouri's most scenic chert feature. Here, Shoal Creek plunges 15 feet over a ledge of solid chert to continue southward. In addition to its novel bedrock and great beauty, Grand Falls holds the Missouri record as our highest, continuously flowing falls. (Missouri has many higher waterfalls, but they don't flow all year.)

So keep an eye out for chert-that common old rock that played a role in human culture and contributes much to Missouri's natural character.

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