In the central Texas region of the Great Plains, which include the
Edwards Plateau, the Llano Basin and the Llano Uplift, the border to the north is the
Interior Lowlands and the border to the south and east is the
Balcones Escarpment.
These two ecoregions, 4K and 4L include the population centers of Austin, Blanco, Brady, Del Rio, Dripping Springs, Fredericksburg, Junction, Rocksprings and San Saba.
Llano Basin
The Llano Basin is underlain by Precambrian rocks and bordered by a discontinuous rim of flat topped hills, which are dissected edge of the Edwards Plateau and consist of overlying Cretaceous sedimentary strata.
Within the Llano Basin and along its margin are down-faulted blocks and erosion surface remnants of Paleozoic strata which form prominent hills.
Llano Uplift
Entirely within the Llano Basin, the Llano Uplift is known for large granite domes such as the Enchanted Rock and Granite Mountain.
The Llano Uplift is a geologic dome, some 100 miles in diameter, which consists of an island-like exposure of uplifted
Precambian eon igneous and metamorphic rocks surrounded by outcrops of the much later Paleozoic era and Cretaceous period sedimentary strata.
These exposed Precambrian uplifted rocks extend about 65 miles westward from the Colorado River valley and beneath a broad, gentle topographic basin drained by the Llano River Basin.
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