The Relationship Between Geology and Soil Development in Austin, Texas. Rob Forkner. Map courtesy of Sue Hvorka, BEG. Mt. Bonnell Fault. View from the top of Mt. Bonnell. Outcrop of the Glen Rose Limestone at Mt. Bonnell. Skimpy soil at Mt. Bonnell.
Development in Austin, Texas
Images from USDA
Shrink/Swell in soil is a function
of seasonality (wet/dry) and
amount of illitic clay within the soil
Map from STATSGO, TX statewide
a more quantitative relationship
between soils and bedrock?
Method Developed by Dr. Mark Helper, UT DOGS
Reclassified geological map with
formations classified into erodability
groups. More resistant rocks have
lower erodability. Limestone=1, Marl=2
Slopes on Austin West DEM formations.
reclassified into 4 groups with 20 degree increments. Steeper slopes are “colder”.
1-20 deg=1; 21-40 deg=2, etc.
Slope calculation applied to
Austin West DEM
Brown area gets 1-1.5 inches more precip. per year.
For erosion weighting, Blue area = 1, brown area = 2
The Combination… formations.
Geology- Limestone = 1; Marl = 2; Alluvium = 3
Slope- 0-20 deg. = 1; 21-40 deg. = 2; 41-60 deg. = 3
60-80 deg. = 4
Precipitation- Most of the map = 1 (appx. 31”); with a small
area in the southeast being 2 (appx. 33”)
VIOLA! Sum of Raster Data formations.
Calculation of “erodability”
as a function of bedrock,
precipitation, and slope.
darker areas are more readily
In general, rock on the upthrown
side of the fault is less erodable
than the rock on the downthrown