The study area lies in the Lampasas Cut Plain within the Grand Prairie Physiographic Province.  The Lampasas Cut Plain is the dissected dip plain of the Edwards Formation that is presently only recognized at the remnant summits of mesas and butts in the area.  The remnant circular flat-topped mesas and buttes form the divides between the drainage valleys.  The largest continuous area of the Lampasas Cut Plain is along the western border of the Grand Prairie, extending from the Colorado in western Travis County to the northeast corner of Comanche County. Within the cut plain, the valley walls of the Lampasas River and its tributary creeks where not covered by Pleistocene and Holocene sediments, expose the upper portion of the Glen Rose Formation underlying the Edwards Formation.  Holocene terrace deposits fill portions the lower elevations of theses valley and unconformably overlie the eroded surface of the Glen Rose Formation.

View Westerly (Down Stream) of a Portion of  the Study Area

Where a 30 Foot High South Cut-Bank of the Lampasas River is Exposed

Note that the Displaced Boulders along the River Bank are from

Terrace Deposit Composed of Indurated Sandy Gravel with Pebbles

Namely these are from oldest to youngest, the T1 (subdivided into the T1a, T1b and T1c) terrace surface underlain by the Georgetown, Fort Hood and West Range alluvial deposits and T0 terrace surface underlain by Ford alluvial channel fill deposits of the Lampasas River.  Just east of the cut bank section is the Pleistocene T2 terrace underlain by the Jackson alluvial deposits.  This is shown in the following generalized cross section.

Generalized Cross Section Across Lampasas River Flood Plain

3 Miles North of Adamsville, Texas

According to work performed by Lee C. Nordt, 1992, the stratigraphic deposits at FortHood in Bell and Coryell counties are informal subdivisions.  The following table summarizes the terraces and stratigraphic deposits as presented in Lee C. Nordt’s report at Fort Hood, Texas. 

According to Michael B. Collins and Thomas R. Hester, 2008, the Georgetown and Fort Hood sediments correlate roughly to sediments at the Clovis age “Gault” archeology site, in Williamson County, Texas.  Only sediments correlating to the Georgetown contained Clovis age artifacts.

In the study area as shown in the following figure, the T2 terrace surface lies between elevations 1,100 to 1,120 feet above MSL.  However, is not exposed along the cut bank section of the Lampasas River.  It is exposed along Highway 281, approximately ¼ mile from the river cut bank exposure.  Based on outcrop observations, the Jackson alluvium maximum thickness is approximately 20 feet thick.  The Jackson consists of sandy gravels unconformably overlying the Glen Rose Formation and interbedded with silty sand and silty clay.

The T1 terrace surface lies between elevation 1,075 and 1,100 feet above MSL.  Based on outcrop observations the Georgetown deposits maximum thickness is 20 feet thick and the FortHood Alluvium maximum thickness is 25 feet.  The Georgetown and FortHood Alluvium are both exposed in the Lampasas River cut bank section as shown below.

Lampasas River South Cut Bank Section

T1 Terrace Exposing Georgetown and FortHood Alluvium

The Georgetown Alluvium consists of yellowish brown to brownish gray, well sorted, sandy gravel unconformably overlying Glen Rose limestone/marl. 

Georgetown Alluvium unconformity with the Glen Rose Formation

The gravels are abruptly overlain by massive, yellowish brown, silty fine sand with a trace of clay (loam).  The Georgetown is capped by the Royalty palesol, characterized by brown silty fine sand and silty fine sand with some clay.

Georgetown Alluvium Contact with Overlying Fort Hood Alluvium at

Royalty Palesol

Fort Hood Alluvium on Cut-Bank of South Side of  River

Note 3 Foot Gravel Layer in Center of  Picture

The West Range alluvium is only exposed on the 15 foot high bank at the west end of the cut bank next to a meander bend of the river.  Here the West Range is erosionally inset to the Fort Hood alluvium and consists of basal silty sand with some gravel, overlain by interbeds of brown to grayish brown silty fine sand with silty clay and silty fine sand. Further down stream an upper and lower West Range terrace is recognized. The lower West Range scarp is approximately 5 feet high, consisting of gray silty fine sand with some clay. The upper West Range scarp is approximately 150 feet away from the lower West Range scarp and consists of similar soils as the lower West Range, except for a 3 foot gravel layer sometimes exposed at the base of the scarp. Approximately 50 yards further down stream the two terraces converge forming a 15 foot scarp. 

Fort Hood Alluvium With West Range Alluvium

West Range Alluvium Gravel in 5 foot Cut-Bank of  North Side of  River

15 foot section of Upper and Lower West Range Alluvium on bank of river.

The modern river alluvium, termed Ford alluvium, forming the T0 terrace surface, consists of fine to medium sands and sandy gravel with some pebbles in the river channel and is erosionally inset to the older terraces.  The Ford surface lies approximately at elevation 1,060 feet above MSL in the study area.  In places, thin almost indisguisable overbank deposits of sand to clays overly portions of the T1 terrace surface.  These are more prevalent closer to the modern river.  In other areas the Ford deposits have formed its own scarp inset into the West Rage alluvium.  The Ford terrace is approximately 10 feet higher than the present day river channel and where present at a maximum, 30 feet wide.

Ford Alluvium Longitudinal Channel Deposit 

Stratigraphy

The Jackson Alluvium is a late Pleistocene terrace that was deposited towards the end of the Wisconsin glacial period when continental glaciers were receding.  This was during a wetter and moister period than previously, where low energy channel and overbank sediments were deposited.

The Holocene Georgetown Alluvium was a period of valley cutting and subsequent valley filling where high energy channel and overbank sediments were deposited.  Some investigators believe the Georgetown valley cutting was performed earlier, not during Georgetown time.  Lee C. Nordt, 1992, stated that the development of the Royalty Palesol was a period of stability lasing 800 to 900 years followed by a period of erosion at the end of  the Georgetown. This was a period of transition from the late Pleistocene wet conditions to even  a cooler and moister climate.

Larger piece of gravel on lower right is 1½ inch in diameter

The Fort Hood Alluvium consists of high energy channel (braided stream) and over bank deposits during a warmer and drier (Atlithermal) period where erosion of upland soil during seasonal events was taking place.

The West Range Alluvium also consists of high energy channel (braided stream) and over bank deposits during a period with greater moisture and increased stream flow with accelerated erosion of upland bed rock.

The Ford Alluvium consists of channel deposits in the modern Lampasas River channel and overbank deposits on the previous terrace deposits, mainly T1, when the climate was warmer and drier than the West Range climate.

All Holocene alluvium is deposited in their alluvial valleys by down cutting (entrenchment) into older terraces and the Cretaceous bedrock.  The alluvium is characterized by sediments eroded from older terraces and upland surfaces, including Cretaceous Formations.  Typically the channel and point bar deposits consist of subround to round quart and limestone sand with a predominance of limestone in the gravels with reworked Cretaceous age fossils (mainly mollusks).  Layers up to three feet thick in the Georgetown may be indurated with lime cement.  High to low angle cross bedding is common in the lenticular channel deposits where gravel unit may be stacked.  In the Ford Alluvium the channel deposits are confined to the modern river channel with little to no down cutting.

Water Worn Cretaceous Fossils from the Ford Alluvium