South of the San Saba River crossing, outcrops are gray limestone of the uppermost Cambrian, overlain by Ellenburger group rocks of Ordovician age. From the San Saba River to Brady the highway crosses Ordovician strata. Brady itself lies on the southern tip of a band of Pennsylvanian age rocks, which form the bedrock over a wide area to the northeast.
A site to investigate is the Wilberns fm. cyanobacterial reef deposits on the San Saba River and highway 87 south of Brady. Stromatolites in this formation are also described in the White's Crossing to James River section of the River Excursions.
Northwest of Brady, in the town of Paint Rock, excursions can be arranged to see pictographs painted on the limestone cliffs bordering the Concho River. The paintings span a time ranging from prehistoric to the end of the nineteenth century.
The trip from Llano to Pontotoc is
presented
with circles on the highway as mile markers and squares as the locations of outcrops and items of interest.
mile 0.0 (30o 45.590; 98o 40.510) - Junction of highways 29, 16 and 71
mile 1.0 (30o 45.647; 98o 41.522) - crossing Pecan Creek; driving on Packsaddle Schist, outcrop in field on left.
mile 2.8 (30o 46.160; 98o 43.202) - highway 71 to Brady on right
mile 3.4 (30o 46.643; 98o 43.504) - distant view (to the east) of Town Peak , a grey colored Town Mountain Granite outcrop
mile 3.6 (30o 46.784; 98o 43.665) - outcrop of Packsaddle Schist on the left
mile 4.5 (30o 47.291; 98o 44.513) - Outcrop of Town Mountain Granite on the left.
mile 5.0 (30o 47.448; 98o 44-840) - outcrop on left is Town Mountain Granite; leave TMG and drive on Valley Spring Gneiss.
mile 6.3 (30o 48.147; 98o 45-806) - Willow Creek.
mile 6.6 (30o 48.366; 98o 46.026) - fence & quarry on left. About at contact between TMG & VSG.
mile 7.1 (30o 48.699; 98o 46.369) - outcrop left VSG, at about 7 miles Valley Springs Gneiss contact.
mile 7.9 (30o 49.159; 98o 47.022) - VSG left in photo , outcrop both sides.
mile 8.2 (30o 43.292; 98o 47-208) - outcrop right is VSG. The extracted section of the geologic map shows the complex relationship between the preCambrian rocks and the Cambrian Hickory Sand.
mile 9.6 (30o 50.080; 98o 48.321) - the outcrop on the right is mapped as "younger granite" - preCambrian intrusion, but younger than the TMG.
mile 9.7 (30o 50.151; 98o 48.404) - photo left , outcrops both sides for about 1/10th mile
mile 11.2 (30o 51.302; 98o 48.993) - Valley Spring. The town is on Hickory Sandstone, then a fault brings back VSG.
mile 12.6 (30o 51.860; 98o 50.106) - outcrop left - Hickory from about mile 13.2 (at fault boundary with VSG to fault at mile 17.5; about dozen ne-sw trending faults.
mile 16.2 (30o 52.494; 98o 63-441) - outcrops of
Hickory Sandstone
Member of Riley Fm. (Cambrian) on both sides of road. This is separated from the VSG by a fault.
Rocks of the Hickory Member of the Riley Formation represent the earliest transgression of the Llano area during the Phanerozoic. The thickness of the Hickory, which consists primarily of coarse-grained, crossbedded quartz sandstone and interbedded, burrowed siltstone, varies substantially across the Llano area due in large part to the irregular upper surface of the underlying Precambrian. The age is in the Bolaspidella trilobite zone of late Middle or early Late Cambrian age.
Only the upper part of the sequence, approximately 5.5 m of the Hematitic Sandstone Facies (alternating units of festoon-bedded and bioturbated sandstone) and 12 m of overlying Even Bedded Sandstone Facies (predominantly burrowed fine-grained sandstone and siltstone), is exposed. Hematitic sandstones are poorly sorted and contain linguloid brachiopods and abundant hematite ooids that are nucleated around a variety of skeletal and siliciclastic grains. These deposits have been interpreted to represent migrating tidal channels.
mile 16.4 (30o 52.495; 98o 53.481) - cliffs of Hickory Ss on both sides of the road for about 1/4 mile
mile 17.0 (30o 52.706; 98o 54.187) - Cold Creek, photo right side. Quaternary alluvium but also Hickory Ss boulders.
mile 17.8 (30o 52.696; 98o 54.984) - Spring Valley Gneiss outcrop on left side of road.
mile 22.5 (30o 54.568; 98o 58-649) - Back into the VSG until Pontotoc at highway 501. Photo of former academy on south side of highway 71.
This small town has had an interesting history since its settlement in 1859.
The trip from Llano Pontotoc to Highway 87 near Brady is presented with circles on the highway as mile markers and squares as the locations of outcrops and items of interest
mile 23.5 (30o 54.540; 98o 59.670) - outcrop on left side
mile 25.7 (30o 54.930; 99o 01.716) - channel in Hickory Sandstone on right.
mile 26.2 (30o 55.233; 99o 02.031) - county line
mile 26.3 (30o 55.260; 99o 02.060) - Hickory both sides
mile 29.8 (30o 55.831; 99o 05.440) - Hickory distant right
mile 30.1 (30o 55.901; 99o 05.737) - outcrop both sides Wilberns; couple of faults
mile 30.8 (30o 55.940; 99o 06.438) - outcrop both sides; Wilberns San Saba stromatolite
mile 31.1 (30o 56.021; 99o 06.795) - Hickory both sides
mile 31.3 (30o 56.093; 99o 06.991) - intersection with 336
mile 33.3 (30o 57.810; 99o 07.051) - preCambrian on right
mile 36.3 (30o 59.414; 99o 09.129) - mining
mile 36.7 (30o 59.780; 99o 09.158) - quarry , mining operation on left
mile 40.3 (31o 00.597; 99o 12.422) - Cambrian Wilberns; stromatolitic bioherms on both sides of road
mile 40.7 (31o 01.017; 99o 12.648) - San Saba River
mile 42.5 (31o 01.941; 99o 13.925) - outcrop (Wilbern) both sides Fault between Tanyard and Wilberns at about 42.2 mile
mile 43.0 (31o 02.268; 99o 14.314) - outcrop (Tanyard) both sides.
mile 44.3 (31o 03.098; 99o 15.248) - outcrop (Tanyard) both sides
mile 44.9 (31o 03.443; 99o 15.627) - outcrop
both sides
mile 48.8 (31o 05.438; 99o 18.746) - outcrop both sides. Marble Falls Limestone (Pennsylvanian).
mile 49.7 (31o 05.730; 99o 19.580) - intersection with hwy 87
Fossil site in Brady (at junction 87-190) has been paved over, so turn left on 87 to go to the San Saba River Wilberns stromatolite.
mile 57.3 (31o ; 99o ) - cross bridge over the San Saba River and pull into rest area on left. This is stop 4 of the 1987 SEPM field trip - the Wilberns San Saba member stromatolites. Turn into the parking area south of the river.
The reef complex is made up of a lower, more continuous biostrome (several hundred meters wide and at least 4 m thick) and an upper zone of elongate bioherms approximately 20-25 m wide and at least 70 m long. Buildups are separated by 1-15-m-wide grain stone- filled channels , some of which are over 6 m thick. The average trend of the large reef "spurs" is N28E, whereas contained stromatolitic columns are oriented oblique to this master trend (N50E), and wave-ripple axes in interreef areas are oriented approximately perpendicular to this trend.
Both the lower biostrome and the elongate bioherms are made up of 20-100 cm diameter discrete stromatolite columns that are a minimum of 50 cm high. Most but not all stromatolite columns are distinctly elongate, with average trends slightly oblique to the trend of the elongate bioherms. Internal structures of the stromatolite columns include the typical convex-up lamination, distinctly more dense outer walls, and selectively dolomitized burrows (borings?) that are ubiquitous throughout this shallow-water subtidal setting. The simple convex-up internal lamination fabric and abundant bioturbation observed in these stromatolites are in marked contrast to internal structures present in the deeper-water bioherms of the Llano River White's Crossing outcrop.
The importance of the elongation developed in these bioherms is as a general indicator of high-energy conditions at the time of growth and as an indicator of shoreline trend. Many studies of similar reef structures to date have indicated that elongation of these large stromatolite-reef spurs is perpendicular or at a high-oblique angle to shoreline trend, being sculpted by the action of wave surge and tidal currents. This relationship, combined with regional depositional trends and isopachs, would suggest that at least the local depocenter was to the southwest.
Stromatolites are typically thought of as organo- sedimentary structures formed by trapping and binding action of filamentous blue-green algae. This perception was largely developed through work on Recent stromatolites from places like Shark Bay, Western Australia, and the Bahamas /South Florida area, and the application of this work to ancient stromatolites, largely of Proterozoic age.
The reef structures and bioherms at the San Saba and Llano rivers (White's Crossing) areas are not stromatolites in the true sense, because they owe their accretion in large part to syndepositional calcification of the organic sheaths of the algae (now more properly referred to as cyanobacteria), and thus represent in situ precipitated growth forms more similar to the reefs of the Phanerozoic.
At the northern end on the east side of the road is an area of darker rocks, where the fossils are most abundant. The ground here looks much as you would expect the floor of the sea to appear after a mass explosion. The fossils are so abundant, that in places, the soil beneath is almost completely hidden.
The specimens here are Pennsylvanian, chiefly crinoids, corals, and bryozoans. While crinoid stems account for the largest portion of the mass, careful searching could produce crinoid heads, cephalopods, and trilobites.