Experimental evidence suggests the detrital origin of underclays of the Illinois Basin. Flocculated clay materials were deposited in glass tubes and the resulting clay-mineral orientation determined by X-ray techniques. Random orientation in the flocculated clay was preserved in the dried clay, which had dewatered slowly under minimal overburden pressure. Samples were compressed to study the influence of water content and pressure on the preservation of random orientation. Clay with lower water content showed less reorientation due to compression than clay with a higher content, suggesting that the amount of liquid water in proportion to adsorbed water in the system may influence clay-flake orientation.

The soil concept in explaining the origin of underclay is discussed. Evidence indicates that the nonbedded character of underclays is due to the random orientation of clay flakes, which could be a primary feature resulting from the slow deposition of flocculated clay during which the clay dewaters under minimal overburden pressure. Observations by the author and others do not support the concept that most underclays in the Illinois Basin are soils subjected to subaerial weathering and reworking by plant rootlets.

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