How do Wichita granite and outwash affect Lawton-area foundations?

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

In short: The Wichita Mountains near Lawton are made of ancient granite and related igneous rock, and streams have spread coarse granitic sediment, called granite wash or outwash, over the red-bed plains around them. Soils formed in that outwash over red clay, such as Foard, are sodium-rich, very slowly permeable and prone to shrink-swell. Around the mountains, it is these clay soils rather than the granite that drive most foundation movement.

What is the Wichita granite?

The Wichita Mountains are built of granite and similar igneous rocks that formed about 525 million years ago ¹. They were uplifted into mountains about 315 million years ago, during the same mountain-building episode that raised the Arbuckle and Ouachita ranges ¹. Igneous rock forms from cooled molten material; granite and gabbro are common examples.

What is granite wash?

As the mountains eroded, streams carried granitic sand and gravel outward onto the surrounding lowlands. This is called granitic outwash or granite wash. USDA soil descriptions record granitic outwash from the Wichita Mountains as part of the parent material of the Foard soil, lying over clayey Permian red-bed sediments ².

What soils are found in the Lawton area?

Foard

A natric horizon is a subsoil layer high in exchangeable sodium. In Foard, exchangeable sodium reaches roughly 15 to 25 percent in the lower subsoil ². Smectitic means the clay is dominated by smectite, the most swelling clay family. Slickensides are polished shear faces in clay left by repeated swelling.

Neighboring clay soils

Other extensive clay soils of the same Red Plains area include:

• Hollister, formed in Pleistocene calcareous clays over Permian sediments, with cracks more than 30 inches deep that stay open 150 or more days in most years ³.
• Tillman, formed in alluvium from Permian red-bed clays and claystone, with cracks deeper than 48 inches ⁴.

What does this mean for foundations?

Strong shrink-swell

Smectitic clay with wide cracks and slickensides in Foard and Hollister ²³, and deeply cracking clay in Tillman ⁴, shrinks during dry periods and swells when wet ⁵. In this setting, slab edges can drop in drought and rise in wet spells. See perimeter drying settlement, perimeter wetting heave and cracks that open and close seasonally.

Slow drainage

Foard and Hollister have very slow permeability ²³, so water from roofs, sprinklers or leaks tends to pond at the surface or soak in unevenly. The ASCE homeowner guide recommends sloping soil away at least 3 inches in the first 5 feet, keeping sprinkler heads at least 5 feet from the foundation and watering evenly on all sides ⁵. See water ponding near the foundation.

Variable outwash

Because outwash was spread by streams over red beds ², the thickness of sandy or gravelly material over clay can change across a neighborhood. Lot-specific soil maps and, where needed, borings show what is actually under a house. See soil testing and lab values.

Related

• Southwest (Lawton and Red Plains)
• Foard soil
• Western Permian red beds
• Clay mineralogy
• Oklahoma geologic provinces