The relevant geology for human habitation on Brushy Creek begins somewhere between the Edwards Formation (Ked) deposition ( Late Cretaceous) and the earliest local Cenozoic deposits (Ica, Ice and Igl).

Some unknown amount time after the diagenesis of the Ked limestone in the Late Cretaceous, a prolonged hot dry climate existed locally, as evidenced by 8-12” of hard, cemented caliche (Ica) riding uncomformably on the Ked stratum, at Brushy Creek’s Southern valley margin. Overlying this Ica stratum is 2-3” stratum of cemented alluvial gravel and sand (Ice):

Figure 9. Highlighted close up of Ked, Ica, Ice and Igl.

Figure 10. Close up of Ked, Ica, Ice and Igl.

Figure 11. Ked is in the foreground with Ica uncomformably overlying the Late Cretaceous stratum. Igl is overlying Ica in the background. In this area of the column the Ice stratum appears to have been eroded off the underlying Ica stratum. Marking the end of this drought is significant valley scouring of Ked and rapid, uncomformable deposition of a thick stratum of buff gravel (Igl) upon the stratum of cemented gravel and sand (Ice), caliche (Ica) and underlying Edwards (Ked).

Figure 12. Valley scouring of the Ked stratum Ca. 100 meters upstream from Lima. View is to the Northeast. Note the remnants of Ica on the Southern valley margin (to the right in this photo).

Figure 13. Dissected Igl, Ice and Ica strata.

Figure 14. Complete column illustrating the Igl stratum within the context of the entire sequence.

The ca. 3’ strata overlying the Igl stratum are several thin alternating layers of silt and much smaller pebble sized gravel (Isi). The upper 3-4’ strata in the profile is soil which consists predominantly of black clay loam and silt (Icl). More than 70 radio carbon assays from Wilson-Leonard indicate the following time periods for each strata’s original deposition:

1. Edwards Formation Limestone Bedrock (Ked): > 12,000 BP- PreClovis
2. Caliche (Ica): > 12,000 BP- PreClovis
3. Cementation Strata (Ice): > 12,000 BP- PreClovis
4. Buff Gravel Stratum (Igl): >12,000 BP- PreClovis
5. Alternating layers of silt and small pebble gravel (Isi): >11,200 BP-Clovis/ Folsom time frame
6. Clay and Silt Topsoil (Icl-IIIc): 11,200 BP to present. Again, the most complete cultural sequence at any single site in Central Texas and one of the more complete in North America.

As discussed earlier, this author’s research is focused on the lower four strata, beginning with the Ked limestone and ending with the upper portions of the Igl stratum. Hence, the title for this report begins: “Starting at Rock Bottom”.