Sections measured in the Franklin and Organ mountains show the Lower Ordovician El Paso formation to have been progressively truncated toward the north, and to be overlain disconformably by the Middle (?) and Upper Ordovician Montoya group.

This group from the base up is divided into: the Cable Canyon sandstone (coarse, orthoquartzitic) replaced in the Baylor Mountains by a St. Peter sandstone equivalent; the Upham formation (dolomite or limestone) containing a Red River fauna, which is probably upper Trenton or possibly lower Cincinnatian in age; the Aleman formation (also dolomite or limestone with much chert, in bands or nodules) with a Maquoketa fauna, notably the coral Palaeophyllum thomi, which crops out in a marker bed for 140 miles; and the Cutter formation (non-cherty dolomite) with a late Ordovician fauna near its base.

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