A dark, finely grained rock that has crumbled from natural processes and lies scattered across the ground is known as traprock, especially when it is composed of basalt, dolerite, or another igneous material. Naturally formed traprock often paves the surface of canyons and valleys. The term is also applied, however, to rock crushed into stones an inch to several inches in size and sold as a building material; this is carefully placed beneath downspouts and in other key positions in “French drains” to absorb the flow of water. The word derives from trapp, Swedish for stair step. Basalt is an extrusive rock that often forms large flows; trap or trapp refers to the steplike structure occasionally seen at the end of such flows. Some of the most impressive examples are found in southeast India at the Deccan Traps, an area of crumbled lava that covers two hundred thousand square miles. Smaller-scale examples are found in the American Southwest. In Cormac McCarthy’s All the Pretty Horses, two characters crossing a basalt landscape in Cook County, Texas, ride “through broken hills dotted with cedar where the ground was cobbled with traprock.”

Jan DeBlieu