What Cretaceous rocks and soils lie under southern Oklahoma?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: Southern Oklahoma's Cretaceous rocks are alternating layers of clay, shale, marl, limestone and sand. Clay and shale layers weather into some of the most expansive soils in the state, while sand layers weather into loose, well-drained soils. The same neighborhood can have both, so foundation behavior depends on which layer a lot sits on.

What are these rocks and where are they?

The Cretaceous Period ran from about 145 to 66 million years ago, and in the Late Cretaceous most of Oklahoma was covered by a shallow sea ³. Rocks from that time are preserved in southern Oklahoma, including the Tishomingo and Atoka areas ¹².

Early USGS mapping of the Tishomingo and Atoka areas described the Cretaceous section as about 1,000 feet of nearly flat layers dipping gently to the south ¹². From oldest to youngest, the units named included ¹²:

Marl is a soft, limy clay. Hard limestones tend to form ridges and terraces, while softer clays and sands form valleys, slopes and rolling uplands ¹.

Why do they form two very different soils?

Durant: clay that cracks deeply

The Durant soil formed in clayey material weathered from Cretaceous shale and claystone, with its typical profile in Bryan County, Oklahoma ⁴. Its clay is smectitic, meaning it is dominated by smectite, the clay mineral family that swells most when it takes up water ⁴. In dry periods Durant develops cracks wider than 1 centimeter that reach about 30 inches deep ⁴. Permeability is very slow and runoff is high ⁴.

Soil scientists rate shrink-swell by linear extensibility, the percent a soil's length changes between moist and oven-dry. Values of 6 to 9 percent are rated high and 9 percent or more very high ⁶. Map-unit values for a specific lot are listed in the USDA Web Soil Survey.

For a house, that means the soil under the slab edge can shrink away in summer and swell back in a wet spring. See perimeter drying settlement and cracks that open and close seasonally.

Bernow: sandy and well drained

Bernow soils formed in sandy and loamy residuum from Cretaceous sandstone, mainly in eastern Oklahoma ⁵. They are well drained with moderate permeability and occur on slopes up to 12 percent ⁵. Sandy soils like this swell little but are easier for running water to carry away, so leaks and roof runoff matter more than seasonal swell. See erosion and washout voids.

What are the practical points for homeowners?

• Look up the soil series for the lot. A clay series like Durant and a sandy series like Bernow call for different explanations of the same crack ⁴⁵.
• On the clay soils, keeping moisture even around the foundation is the main lever. See root barriers and moisture control.
• On sandy soils, watch for soil loss at slab edges and downspouts. See surface drainage and grading.

Related

• South-Central (Arbuckles and Texoma)
• Southeast (Ouachitas and Red River)
• Durant soil
• Bernow soil
• Clay mineralogy
• Shrink-swell and the active zone