Round Rock is a city in the U.S. state of Texas, in Williamson County, which is a part of the Greater Austin, Texas metropolitan area. The population was 99,887 at the 2010 census.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 north of downtown Austin, Round Rock shares a common border with Austin at Texas State Highway 45.In August 2008, Money magazine named Round Rock as the seventh-best American small city in which to live. Round Rock was the only Texas city to make the Top 10. In a CNN article dated July 1, 2009, Round Rock was listed as the second fastest-growing city in the country, with a population growth of 8.2% in the preceding year.