How do Pennsylvanian shale clays affect Oklahoma foundations?

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

In short: Thick Pennsylvanian shale underlies the lower ground of the Tulsa metro and much of eastern Oklahoma. Near the surface it weathers into clay soils that drain poorly, hold perched water in winter and spring, and in many cases swell and shrink strongly. These soils are behind much of the seasonal foundation movement in the region.

What is Pennsylvanian shale?

Shale is rock made of compacted clay and silt that splits into thin layers. In northeastern Oklahoma, the Nowata is an example: a series of bluish or greenish shales with a few sandstones and at least one coal bed, varying from about 50 to 600 feet thick at Tulsa ¹. Its outcrop is about 8 miles wide east of Tulsa ¹.

Farther south in the Arkoma Basin, a USGS study of the Boggy Formation near McAlester described brown to dark-gray shale with numerous thin sandstone beds, 610 to 1,220 meters thick ². Key findings from that study ²:

Fractures were most common at shallow depths, and slickensides were generally visible on fracture surfaces.
About 2 meters of clay-rich soil lay over weathered shale at the test site.
Lab and field tests found very low hydraulic conductivity, meaning water moves through the intact shale very slowly.

That last point is why shale acts as an aquitard, a layer that holds water up rather than letting it drain.

What clay soils form on shale?

Series	Parent material	Drainage	Perched water	Notable trait
Dennis	Pennsylvanian shale residuum ³	Somewhat poorly drained ³	12–30 in, Dec–Apr ³	Solum over 60 in ³
Okemah	Alluvium or colluvium from Pennsylvanian shale ⁶	Somewhat poorly drained ⁶	2–3 ft, Dec–Apr ⁶	Slow permeability ⁶
Parsons	Old alluvium or Pennsylvanian shale residuum ⁵	Somewhat poorly drained ⁵	6–18 in, Dec–Apr ⁵	Clayey subsoil, 35–60% clay ⁵
Summit	Pennsylvanian shale colluvium or residuum ⁴	Moderately well to somewhat poorly drained ⁴	12–36 in, Nov–May ⁴	Smectitic; deep summer cracks ⁴
Talihina	Soft Pennsylvanian shale ⁷	Poorly drained ⁷	0.5–2 ft, Nov–Apr ⁷	Rock at 10–20 in ⁷

How much they swell

Soil scientists rate shrink-swell using the coefficient of linear extensibility (COLE), the fraction a soil's length changes from moist to dry. A COLE of 0.06 to 0.09 is high and 0.09 or more is very high ⁸. The Summit description reports deep, wide summer cracks with a COLE of 0.09 or more, and slickensides in the subsoil ⁴. Summit's clay is smectitic, meaning dominated by the most swelling clay mineral family ⁴.

What does this mean for foundations?

Wet-season edge lift. Perched water from late fall to spring ³⁴⁵ wets the clay at slab edges and under crawl spaces. See perimeter wetting heave and groundwater and perched water.
Summer edge drop. As perched water disappears and clay dries, deep cracks open in soils like Summit ⁴. See perimeter drying settlement.
Crawl space moisture. A water table 6 to 30 inches below the surface in winter and spring ³⁵ is within the depth range of a typical crawl space. See crawl space moisture and wood decay.
Shallow shale on slopes. Talihina, on slopes of 5 to 20 percent and more, reaches shale at 10 to 20 inches ⁷. See shale weathering and bearing strata.

Common questions

What is shale and why does it matter for foundations?

Shale is a layered rock made of compacted clay and silt. When it weathers near the surface it turns back into clay soil, and in eastern Oklahoma many of those clay soils swell when wet and shrink when dry.

Why is the ground so wet in winter and spring on shale soils?

Unweathered shale lets very little water through, so rain collects above it as a perched water table. Dennis, Okemah, Parsons and Summit soils all hold perched water in roughly November through May.

Which shale soils swell the most?

Summit is a smectitic clay, the swelling clay family. It forms deep, wide summer cracks and has a coefficient of linear extensibility of 0.09 or more, which is in the very high class.

How thick are these shale formations?

Very thick. The Nowata shale varies from about 50 to 600 feet thick at Tulsa, and the Boggy Formation near McAlester is 610 to 1,220 meters.

Sources

Geologic Unit: Nowata USGS National Geologic Map Database (Geolex)
Geologic and Hydraulic Characteristics of Selected Shaly Geologic Units in Oklahoma (WRI 96-4303) U.S. Geological Survey
Official Soil Series Description: DENNIS Series USDA Natural Resources Conservation Service
Official Soil Series Description: SUMMIT Series USDA Natural Resources Conservation Service
Official Soil Series Description: PARSONS Series USDA Natural Resources Conservation Service
Official Soil Series Description: OKEMAH Series USDA Natural Resources Conservation Service
Official Soil Series Description: TALIHINA Series USDA Natural Resources Conservation Service
Linear Extensibility (definition and shrink-swell classes, after NRCS National Soil Survey Handbook) UC Davis California Soil Resource Lab

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