The Manzano Mountains, on the east edge of the Rio Grande Valley south of Albuquerque, are fault-block mountains tilted to the east. Precambrian rocks that make up the main mass of the mountains are overlain by about 180 feet of clastic rocks assigned to the Sandia Formation of Middle Pennsylvanian age, which in turn is conformably overlain by the Madera Limestone of Middle and Late Pennsylvanian age. Fusulinid faunas indicate that the lower part of the Madera Limestone, about 600 feet thick, was deposited during Des Moines time, and the upper part of the Madera Limestone, about 780 feet thick, during Missouri and Virgil time.

As exposed near Tajique, the upper part of the Madera Limestone consists of three similar sequences of limestone and clastic rocks, designated units B, C, and D, of Missouri, early and middle Virgil, and later Virgil ages, respectively.

Unit C, the best exposed and most typical, consists of a basal channel-like deposit of arkose and siltstone as much as 90 feet thick which, to the east, may be truncated by reef-like masses of limestone. It grades upward into siltstone followed by gray shale that contains local red beds and becomes calcareous toward the top. A limestone unit as much as 140 feet thick conformably overlies the shale. The basal part of the limestone is commonly yellowish gray, poorly bedded calcilutite that contains many algal (?) bodies. The calcilutite grades upward into light olive-gray, well-bedded, bioclastic calcarenite composed in large part of comminuted shell

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debris. In turn, this bioclastic member grades upward into calcarenite composed of particles of limestone. Basal arkose of unit D overlies that calcarenite.

Unit B, insofar as exposed, and unit D show this same sequence of rocks, although there are differences in the proportion of red beds.

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