Round Rock is a city in the U.S. state of Texas, within the Austin–Round Rock metropolitan area and Williamson County. The 2010 census places the population at 99,887. The city straddles both sides of the Balcones Escarpment, a fault line in which the areas roughly east of IH-35 are flat and characterized by having black, fertile soils of the Blackland Prairie, and the west side of the Escarpment which consists mostly of hilly, karst-like terrain with little topsoil and higher elevations and which is part of the Texas Hill Country. Located about 20 miles (32 km) north of downtown Austin, Round Rock shares a common border with Austin at SH 45.