These rocks consist of about 900,000 km2 (350,000 sq mi) of Middle Proterozoic crystalline basement exposed in an erosional window eroded through overlying Phanerozoic sedimentary strata.

Each of these domains contain distinctive rock types and ages and were either erupted, intruded, or deposited in three separate areas and later tectonically juxtaposed during the Grenville Orogeny.

The Packsaddle domain consists mainly of schists composed of hornblende, biotite, muscovite, and actinolite; marbles and calc-silicate rocks; quartzites; and quart-feldspar gneiss.

Coal Creek domain also contains diorite plutons, gabbro, amphibolite, mafic schist, minor talc, and smaller serpentinite bodies, all of which were metamorphosed about 1.26 Ga.

[2][9] The Moore Hollow Group records the advancing of a sea from the southeast across eroded Precambrian rocks during Middle to Late Cambrian times and subsequent burial beneath coastal and nearshore marine sediments.

The Cambrian sea spread northward across the eroded surface an area of Precambrian rocks that had a local relief as great as 240 meters (790 ft).

As a result, sediments composed of locally derived residuum, often wind-abraded accumulated as a thin, discontinuous cobble conglomerate overlying Precambrian strata at the base of the Moore Hollow Group.

Following the deposition of the uppermost Cambrian limestones and dolomites, the Lower Ordovician Ellenburger Group (composed of the Tanyard, Gorman, and Honeycut Formations) accumulated within shallow-water carbonate platforms.

The reworked conodonts and the Burnam Limestone indicate that the region of the Llano Uplift was likely either partially and briefly submerged during the Middle and Upper Ordovician only to have the sediments deposited during these inundations removed by later erosion.

Isolated deposits of fossiliferous Starcke Limestone preserved in ancient sinkholes developed in the Ellenberger Group provide definite evidence of the Llano region having been inundated by marine waters at least once during the Silurian Period.

As the Fort Worth Basin deepened and sank in responses to the Ouachita Orogeny, Smithwick Shale was deposited on former sites of Marble Falls sedimentation within the Llano Uplift region.

In the deeper part of the Llano region, basin-fill shale and submarine fan deposits that form the lower Strawn Group accumulated.

During the Cretaceous, this surface was progressively buried by the accumulation of fluvial and coastal sediments of the Trinity Group and later by the Walnut, Comanche Peak, and Edwards formations.

Over the decades, a few small mines have yielded yttrium and other rare-earth minerals, magnetite, feldspar, vermiculite, serpentine, and gem quality topaz.

Briefly, galena as lead ore was mined from limestone lying unconformably upon granite knobs that were once hills before being submerged by rising relative sea level in the Cambrian.

Minor showings of gold, silver, copper, tin, bismuth, molybdenum, tungsten, and uranium minerals have been found and explored in prospecting pits.

In the past large quantities of soapstone were excavated from outcrops south of Llano, Texas, and ground for use as insecticide carrier and inert filler in various products.