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Boulders of sedimentary rock are present in shale units in the Tesnus and Dimple formations near the western margin of the Marathon uplift in the Payne Hills, a terrane underlain by imbricate thrust plates. Although, locally, boulders have been rolled along the thrusts as "ball bearings," a sedimentary origin as submarine-slope deposits for the beds is inferred because (1) most boulders are unlike rocks involved in thrusting, (2) boulders are not brecciated, and (3) many boulders do not occur along thrust faults.

Boulders range from 1 to 24 ft long and are unevenly distributed along strike. Large boulders lie with their long dimension in the plane of bedding of the host rock. The boulder-bearing unit in the Tesnus is from 5 to 50 ft above the base of the formation; that in the Dimple is about 100 ft above its base. Common boulders are light-colored dolosparite and dolomicrite, mottled, dark-green chert, and chert sharpstone conglomerate; a few boulders are limestone, sandstone, novaculite, and porphyry (Dimple Formation only). The chert resembles, but is not identical with, beds in the Caballos Novaculite, although the other rocks are unlike indigenous Paleozoic strata in the uplift. Early Ordovician, Silurian(?), and Middle Devonian conodonts occur in the carbonate boulders.

The boulders were derived from a positive, tectonic element northwest of the basin that was active intermittently throughout Paleozoic time. The lack of fine debris in the boulder beds suggests that the boulders rolled or slid into place and outdistanced finer detritus.

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