CRYSTAL MOUNTAIN SANDSTONE/FORMATION

Age: Early Ordovician Period
Distribution: West-central Arkansas, Ouachita Mountains (principally Montgomery and Garland Counties); southeastern Oklahoma
Geology: The formation is typically composed of massive, coarse-grained, well-rounded, light-gray sandstone. Lesser amounts of interbedded light-gray to gray shale, black chert, bluish-gray limestone, and gray calcareous conglomeratic sandstone (often containing clasts of meta-arkose) are usually present. Some large boulders of meta-arkose and other exotics occur in some slurried conglomerate intervals. The unit is often set with a network of quartz veins up to several inches thick. In some places the quartz veins are open (up to several feet wide) allowing clusters of quartz crystals to form. Conodonts are present in this formation. The contact with the underlying Collier Shale is considered conformable. Typical thicknesses of the unit range from 500 to 850 feet, but some sites may have less than 50 feet.
Original reference: A. H. Purdue, 1909, Geological Society of America Bulletin v. 19, p. 557; A. H. Purdue, 1909, Slates of Arkansas: Arkansas Geological Survey, p. 30, 32.
Type locality: Named for the Crystal Mountains, Montgomery County, Arkansas