In many places in New Mexico and Arizona a kaolinized, white horizon has been noted at the top of the Morrison formation (Jurassic) immediately under the Dakota (Cretaceous), as well as a kaolin-cemented, white sandstone which is the more prevalent form of the distinctive feature. In Rio Arriba County, New Mexico, there occurs a nearly pure white, massive kaolinite in small lenses at the contact between the formations.

The kaolinization is of such widespread distribution that it may be useful as a marker horizon in drilling operations, and it provides some additional data on the probability of a moist climate prevailing during the period separating the Jurassic and Cretaceous in this area.

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