Basalt flows in the San José Valley, Valencia County, New Mexico, are of pahoehoe type and are characterized by flow-units. A "flow-unit" may be defined as a tongue-shaped structure within a flow. In transverse cross section flow-units are small lenses from ioo to more than 300 feet long and from 10 to more than 20 feet thick. As many as five flow-units, one on top of the other, may occur in a flow. As there is no evidence of erosion, weathering, or deposition between flow-units, the time-interval between them must be short. The simplest flow mechanism for lava is one in which the lava moves as a single unit. The existence of the flow-units, however, indicates a greater complexity in the mechanism of these flows. At the front of the flow and also on its margins tongues a few hundred feet wide break out and flow for considerable distances. A number of tongues form, crusts appear on them, and these tongues are buried by later tongues, which in turn are buried by others. In this way multiple flows are formed. Such flows differ from single flows of pahoehoe type in their greater proportion of vesicles and other cavities, a fact of importance in the movement of solutions through them.
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Robert L. Nichols (1936) studied this question.