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Earth Geology Rock Types Chert

Chert

Chert is a common surface rock that is often a headache to farmers and gardeners as they try to work the soil. Chert also forms the bulk of the gravel bars and beds that clog our streams. 

Chert is a very hard and resistant microcrystalline variety of quartz, SiO2. It is extremely resistant to weathering and remains in the soil that forms from the weathering of dolomite. As the soils are eroded and washed away, the chert remains or gets washed into the streams as gravel bars. 

Chert is usually white to light gray, but can range through shades of red, yellow and brown to a very dark gray. The colors are due to inclusion of iron oxides, organic matter and other materials. Chert in the Ozarks is commonly called flint and was used to make arrowheads because of its glass-like fracture and hardness.

Chert nodule Chert lenses
FIGURE 1
Chert nodule (white) with banding in light to dark grey Jefferson City dolomite. Roadcut on Highway 5 about one mile south of Bryant Bridge. Large nodule is about 4" by 12".
FIGURE 2
Small chert nodules and lenses just below and to the right of Figure 1. Lenses are at one to two inches thick.
Brown chert with quartz Chert layer
FIGURE 3
White to light gray quartz druze in brown chert.
FIGURE 4
Chert layer (white) in gray dolomite. The layer is continuous over tens of feet and varies from two to four inches thick.

Chert typically occurs as nodules or "globs" (Figures 1 and 2) within the dolomites and limestones of the Ozarks. These nodules can have a wide variety of shapes and have been mistaken for "petrified" bones, logs, eggs and animals. Chert nodules may be plain or banded with a variety of colors. Chert often contains small cavities that may be coated with small quartz crystals called druze (Figure 3). When sufficient iron oxide is included in the chert, the color may be a deep red; the rock is then called jasper. Chert is especially abundant within the dolomite layers of the Roubidoux Formation and at times forms continuous layers or ledges of chert (Figure 4) that may be as much as twelve inches thick. These chert ledges are impermeable and are often associated with small springs as the base rock for the overlying permeable layers. 

Chert nodules are formed due to chemical migration of silica during diagenesis: the process of creating rock from sediment. In places, the chert contains structures or textures that show it has replaced a pre-existing sediment as the pile of "mud" became rock. Occasionally fossils can be found in chert; however, fossils are very rare in most of the rocks of the Bryant. The most commonly seen fossil is the spiraled shell of a marine snail or mollusk. 

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