This region of south central Texas is often referred to as the "hill Country" due to its tall rugged hills. The terrain is punctuated by a large number of limestone or granite rocks and boulders and features a number of caverns. The area is called home to several tributaries of the Colorado River of Texas — including the Llano and Pedernales Rivers, which cross the region west to east and join the Colorado River as it cuts across the region to the southeast. The Guadalupe, San Antonio, Frio, Medina, and Nueces rivers originate in the Hill Country. The largest cities in the area are San Antonio
and the Texas capitol city of Austin.