The geology of Texas
The ground under the story. Texas is a stack of tilted sheets, each younger belt lying on the older one and dipping toward the Gulf, so that crossing the state from the Panhandle to the coast walks down through time and up through it again. The colony's ground straddles two of those belts. The Austin Chalk — the soft white limestone that runs from the Rio Grande through San Antonio to Dallas and on to Texarkana — makes the bluff and the escarpment the colonists called Chalk Hill; the Eagle Ford shale lies beneath it and crops out to the west, and the survey's map puts most of the 1858 holdings on the shale, with the chalk at their eastern edge. Émile Remond identified the chalk, Ferdinand Michel burned it for lime for twenty-five years, and it became the cement industry that carried the colony land into the twentieth century.
The state
The USGS compilation's 360 map units, dissolved into fourteen age groups. Hover for the group; the faults are the thin dark lines, the Balcones zone among them. The small ring is the colony.
The Dallas belt
The same data at full detail around Dallas County, every unit by name. The Cretaceous belts run north-east to south-west: the Woodbine sands on the west, then the Eagle Ford shale, the Austin Chalk, and the marls and chalks of the Taylor and Navarro groups to the east, each younger than the last. The Trinity has cut through the chalk and laid its terraces and alluvium in the gap. The colony's holdings and the 1855 town are outlined: the holdings on the Eagle Ford, their eastern parcels reaching the chalk; the town on the river's terrace at the chalk's edge. At this scale the survey generalises — the chalk bluff at Cement City is real, and finer than a 1:250,000 line — so read the belts, not the boundaries.
Hover a unit for its name; click for the survey's description of the rock. Present-day streets can be switched on at the top right.
| Unit | Age | Rock | |
|---|---|---|---|
| Qal | alluvium | Cenozoic | Quaternary | Holocene | sand |
| Qt | Terrace deposits | Cenozoic | Quaternary | Pleistocene Holocene | terrace |
| Qu | Quaternary deposit, undivided | Cenozoic | Quaternary | sand |
| PAk | Kincaid Formation | Cenozoic | Tertiary | Paleocene | sand |
| Kau | Austin Chalk | Mesozoic | Cretaceous-Late [Gulfian] | limestone |
| Kef | Eagle Ford Formation | Mesozoic | Cretaceous-Late | shale |
| Kkc | Kemp clay and Corsicana Marl, undivided | Mesozoic | Cretaceous-Late [Gulfian] | clay or mud |
| Kmb | Marlbrook Marl | Mesozoic | Cretaceous-Late [Gulfian] | clay or mud |
| Kne | Neylandville Marl | Mesozoic | Cretaceous-Late [Gulfian] | clay or mud |
| Knm | Neylandville and Marlbrook Marls, undivided | Mesozoic | Cretaceous-Late [Gulfian] | clay or mud |
| Kns | Nacatoch Sand | Mesozoic | Cretaceous-Late [Gulfian] | clay or mud |
| Ko | Ozan Formation | Mesozoic | Cretaceous-Late [Gulfian] | clay or mud |
| Kpg | Pecan Gap Chalk | Mesozoic | Cretaceous-Late [Gulfian] | limestone |
| Kwb | Woodbine Formation | Mesozoic | Cretaceous-Late [Gulfian] | shale |
| Kwc | Wolfe City Formation | Mesozoic | Cretaceous-Late [Gulfian] | sand |
| Kfd | Fort Worth Limestone and Duck Creek Formation, undivided | Mesozoic | Cretaceous-Early | limestone |
| Kgm | Grayson Marl and Main Street Limestone, undivided | Mesozoic | Cretaceous-Early Cretaceous-Late | mudstone |
| Kpd | Pawpaw Formation, Weno Limestone, and Denton Clay, undivided | Mesozoic | Cretaceous-Early | mudstone |
About the sources
Both maps are the USGS State Geologic Map Compilation for
Texas (Stoeser and others, 2005, updated), which digitised the Bureau of
Economic Geology's Geologic Atlas of Texas — the same lineage as
the printed map in the Texas Almanac. Public domain. The atlas's 35,000
polygons and 360 units are reduced by src/land/geology.py:
for the state, each unit's USGS age string is read into one of fourteen
groups (the script documents the rules; mixed ages go with the younger end,
except at the Cretaceous boundary, where the Washita and Woodbine units go
with the Lower) and the groups are dissolved and simplified. For the Dallas
belt nothing is grouped: every unit inside the box keeps its name, age and
rock type from the survey. Faults are the compilation's structure layer.
The colony outline is the 1858 holdings from the main map; the 1855 town is
the city's historical-limits series.
The colony on its terrain → · The colony lands → · Texas, 1850–1900 →