Two large limestone masses, in the Canyon series of the Pennsylvanian, crop out in north-central Texas. One of these masses crops out in Wise County and has been referred to as the Chico Ridge reef, the other crops out in Palo Pinto County and has been referred to as the Possum Kingdom reef. During the past 3 years these two limestone masses have been the subject of detailed surface mapping, sampling, coring, and laboratory analyses. Both of these limestone masses change laterally into equivalent shale and sandstone. The surface and subsurface evidence indicates that these two bodies of limestone are of non-reef origin. These limestone masses are examples of stratigraphic-trap carbonate reservoirs.

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