From the intersection of highways 16 and 1432, you are traveling across Town Mountain Granite to the town of Kingsland.
The Cap Mountain consists of a lower carbonate interval, a middle siliciclastic interval, and an upper carbonate interval, with the siliciclastic and the upper carbonate intervals at this outcrop. The siliciclastic interval is composed of calcareous, very- fine sand/coarse silt arkose that contains thin, low-angle, mm-scale cross-laminations and is locally bioturbated. Thin-shelled inarticulate brachiopods (well exposed in boulders lining the parking area) are common in these deposits along with fine-grained debris of a variety of other organisms.
The Lion Mountain consists of approximately 16 m of alternating intervals of trough crossbedded, glauconitic sandstone and glauconitic skeletal grainstone. Both units contain abundant trilobites, brachiopods, and pelmatozoan debris. These deposits are tidal inlet deposition, based on the presence of bidirectional crossbedding, uniformly coarse grain size, and abundant skeletal material.
Glauconite sandstone in this formation contains as much as 50% glauconite, most of which is in the form of pellets, although minor replacement of skeletal debris also is common.
From the intersection of highways 16 and 1432, you are traveling across Town Mountain Granite to the town of Kingsland.
A short distance east of Kingsland, the road makes a steep climb to a scenic overlook at the southwest end of Backbone Ridge , where a parking area provides great views up and down Lake Lyndon B. Johnson reservoir on the Colorado River. Across the river to the west is the hilly, granite and schist terrain of the ancient rocks of the Llano uplift; the roadcut at the overlook is a full geologic lesson in itself.
In the roadcut, Cambrian rocks, about 500 million years old, display a variety of sedimentary and structural features. As you walk up and down the roadcut, notice the well-defined faults that offset bedding in several places. At the south end of the cut, you can see tilted limestone abruptly faulted down against sandstone.
The rocks that make up this hill are part of a fault-bounded block of Paleozoic rocks within the Precambrian granite that surrounds them. Here at the top of the hill you can see the Morgan Creek Limestone and the Lion Mtn. Sandstone. Both are Cambrian in age. Up at the top is probably the Welge Sandstone. A little way lower down the hill, you can see the Cap Mtn. Limestone, also Cambrian in age. The Cap and Lion Mtn. units are in the Riley Fm., the Morgan Creek and Welge are in the Wilberns Fm.
The organic richness of the dark black to green sandstones and shales is immediately evident. Pieces of trilobites, those ancient jointed-leg, segmented creatures that look like modern pill-bugs found in your garden, are common in these rocks. The green color of the sandstone is due to the mineral glauconite, an iron-rich mineral deposited in shallow marine waters commonly as fecal pellets of ocean organisms. The trilobites and glauconite thus place the deposition of this Cambrian sand in shallow marine seas.
Whitish crossbeds are also noticeable in the midst of the dark sands. The cross beds represent periods when trilobites moulted, and the moults were swept along by currents as particles to form cross bedded "sand," which was then cemented into hard rock by calcite mineralization.
The most spectacular thing about the outcrop are the many small faults that cut the rock layers. Most of the faults have normal motion, but some have oblique slip. Look for the places where fluid flow along fault planes has oxidized the green glauconite into a reddish crust.
From the Hoover Point Lookout, the road heads down the south flank of Backbone Ridge, and once again traverses the low topography of the Town Mountain granite. Near Marble Falls, granite domes rise above the general plain, and soon giant quarries, their towering cranes and piles of cubic quarried blocks appear near the road.
Granite Mountain has been quarried here since the 1880's, when granite blocks were cut and shipped via a specially constructed railroad line to Austin to build the State Capitol Building. Marketed as "Texas Pink Granite," stones from this quarry grace many buildings throughout the United States and as far away as Iceland and Singapore. Blocks of granite from these quarries were also used to construct the stone groins which extend seaward from the seawall along the coast at Galveston.
Across the highway from the Granite Mountain quarries is a roadside park with granite slabs for table tops. Access to an area north of the picnic grounds, via the stiles over the fence, is open to the public; unlike Enchanted Rock, it is permissible here to hammer on the granite.
Just east of the quarries, heavily vegetated, upended Marble Falls limestone is downthrown against sparsely vegetated Town Mountain granite along a major fault having at least 3,000 feet of displacement. Between the Marble Falls limestone and the center of the town of Marble Falls, the highway rests on topographically low Smithwick shale of lower Pennsylvanian age.
The dark gray limestone seen around Marble Falls is over 300 million years old, lower Pennsylvanian age. The appropriately named Marble Falls limestone, although no true marble is present, forms cliffs in the canyon walls of the Colorado River beneath the U.S. 281 bridge south of town center. The best view is back toward town from the south side of the bridge. These Pennsylvanian rocks, like the Ordovician rocks seen on Backbone Ridge, are blocks caught and preserved between northeast-southwest-trending faults flanking the eastern edge of the Llano uplift.
mile 0.0 (30o 43.990; 98o 27.960) - start this trip at the intersection of highway 29 and 1431 - 15 miles east of Llano. The road is on Town Mountain Granite to Kingsland.
mile 2.9 (30o 41.543; 98o 27.635) - granite outcrop on left, beyond railroad track.
mile 3.2 (30o 41.225; 98o 27.650) - turnoff to highway 3404 on the right. Turn to the right and drive about one mile to see the low water crossing over the Llano River (called the slab). This milage not accounted in the road log.
mile 6.4 (30o 39.486; 98o 25.752) - cross the Colorado River; the Llano River (Lake Lyndon B. Johnson) can be seen to the right
mile 7.7 (30o 38.707; 98o 24.950) - scenic outlook & parking. This is the Hoover Point outcrop.
mile 16.1 (30o 35.549; 98o 17.949) - Granite Mountain quarry (to the right) from roadside park on left.
mile 16.8 (30o 35.225; 98o 17.401) - outcrop of Town Mountain Granite on left.The fault contact of the granite and sedimentary rocks is between this outcrop and the next outcrop of limestone.
mile 16.9 (30o 35.187; 98o 17.353) - steeply dipping outcrop of Marble Falls Limestone on the left.
mile 18.1 (30o 34.707; 98o 16.392) - highway intersection 1431 & 281. If you turn right and cross the bridge there is a view of Marble Falls Limestone (Pennsylvanian) from the south side of the bridge over the Colorado River.