The Ground Water Branch of the United States Geological Survey is making an investigation of the ground-water resources of the Navajo country. The recognition of intertonguing, lateral gradation, and facies changes within strata of Jurassic age is essential for exploration of water in the Navajo country. One of the major physiographic features of the region is the Black Mesa basin in northeastern Arizona.

Several Upper Jurassic formations in the area grade laterally and intertongue southwest into a distinct eolian-sandstone phase, which the writer has named the Cow Springs sandstone. This sandstone was deposited in the southern part of the region at the same time that subaqueous Upper Jurassic strata were being laid down in the northern part of the region. The type locality of the Cow

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Springs sandstone is on the west side of the Black Mesa basin, where the formation consists of greenish gray well sorted cross-stratified firmly cemented quartzitic sandstone. This formation extends from northwestern New Mexico to south-central Utah and attains a maximum thickness of nearly 500 feet in the southern part of the area.

The Upper Jurassic strata that comprise the northern phase of deposition include the Entrada sandstone, Summerville formation, and Bluff-sandstone member and the other members of the Morrison formation. A better understanding of lateral gradation and intertonguing of these rocks with the southern eolian sandstone phase is dependent upon the recognition of contemporaneous eolian and subaqueous environments.

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