The cap, or hard layer, underlying the Llano Estacado is a major geological feature known to Texans as the Caprock. It is not a rock layer in the usual sense of the term but is more technically a "hard-pan" layer that developed a few feet below the ground as highly mineral subsoil particles cemented themselves together to form a rock-like layer that resists erosion. Although the name Caprock technically applies only to the formation itself, the expression is often loosely used to mean the whole Llano Estacado. The Caprock escarpment was formed by erosion about one million to two million years ago. Prehistoric nomadic hunters, Plains Apaches, and Comanches lived in the region. The Spanish explorer Vásquez de Coronado traveled the area in 1541. The Caprock escarpment forms a natural boundary line between the High Plains and the lower rolling plains of West Texas. It stretches from the Panhandle into Central Texas and can be seen most prominently in Briscoe, Floyd, Motley, Dickens, Crosby, Garza, and Borden counties, where it reaches its highest elevations, rising abruptly above the plains at 200, 500, or as much as 1,000 feet. The east-facing wall is often cut by rivers, forming canyons such as Palo Duro Canyon.