The asphaltic-base crude gradually thickened until only a heavy tar or asphalt remained. The bitumen content of commercial rock asphalt varies from 3 to 15 percent. Rock asphalt was once mined on a large scale in Kentucky. Large deposits occur in Edmonson, Grayson, and Hardin Counties in western Kentucky, and Carter County in eastern Kentucky. Periodically and most recently during the early 1980's, attempts have been made to reclaim the petroleum in the rock asphalt by heat treatment, distillation, and other processes.
Rock asphalt is used principally for surfacing streets and highways.
Other uses are roofing, waterproofing, flooring for factories, and blending with rubber. Contact the Webmaster Shale Shale is the most abundant of all sedimentary rocks. It is composed primarily of soft clay minerals, but may include variable amounts of organic matter, calcareous material, and quartz grains. Shale may be any color, but is generally greenish gray to grayish black. It is relatively soft and has a smooth, greasy feel when freshly exposed, but is hard and brittle when dry.