White's Crossing to James River

River Section

Take highway 1871 from Yates or from Mason and drive to White's low water crossing on the Llano River. Park on the northeast side of the bridge to launch Kayaks. The first 100 yards of the river has to be pulled, the water is to shallow to float.

The rocks exposed along this part of the river are upper to middle Cambrian of the Moore Hollow Group . This group includes the Riley and Wilberns formations

Deeper-water outer-ramp series of spectacular large bioherms of the Point Peak member of the Wilberns Fm. comprising calcareous algae exposed in a Member, Wilberns Formation, striking yet serene cutbank on the Llano River near White's Crossing.

Tracing of classic "White's Crossing-type" Point Peak bioherm exposed along the Llano River. Features of interest pointed out in the sketch are the roughly 60% compaction of interbioherm shales

syndepositional compaction of interbedded limestones and shales beneath the bioherm
an intrabioherm channel
Holocene (?) tufa occurs at the lower right
Synoptic relief of approximately 5 m is defined by tracing one of the continuous growth horizons between "a" and "b"

A vertical succession of lithologies from wave-rippled, hummocky cross-stratified siltstone, sandstone and shale, through interbedded limestone and shale, and finally shaly limestone is seen in this upper portion of the Point Peak

Facies surrounding the White's Crossing bioherms are siltstones, shales, and sandstones displaying wave rippling, hummocky bedding, and storm-generated rip-up clast beds, in contrast to the high-energy carbonate grainstones of the San Saba reefs on highway 87. The planview of these bioherms is circular rather than the elongate morphology of the San Saba type. This can be seen in a large boulder that has fallen into the river

Synoptic relief on the White's Crossing mounds was up to 5 m, whereas the San Saba bioherms probably maintained no more than 1-2 m of relief above the migrating bioclastic channel fill. Hummocky cross-stratification within the interbioherm calcareous sandstones also indicates a moderate- to high-energy shelf setting. The rip-up clasts within these storm deposits are interpreted to represent reworked incipient hardgrounds that formed at or near the sediment-water interface.

Travel Log

(30^o ; 99^o ) - start at southeast end of White's Crossing bridge. IPsw at road

(30^o ; 99^o ) - cross fault, then outcrop 1; formation ?

(30^o ; 99^o ) - outcrop 2; formation ?

(30^o ; 99^o ) - large bioherms in the Wilberns Fm. (Point Peak)

(30^o ; 99^o ) - outcrop 8; formation ?

(30^o ; 99^o ) - outcrop 9; formation ? on north side of river.

(30^o ; 99^o ) - outcrop 12; formation ?

(30^o ; 99^o ) - cross fault, then outcrop 13; formation ?

(30^o ; 99^o ) - Hickory Ss ? or Cap Mountain on east bank of James River at cofluence with the Llano River

(30^o 38.983; 99^o 15.031) - James River Crossing highway 2389.

return to top