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The Relationship Between Geology and Soil Development in Austin, Texas

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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.

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The Relationship Between Geology and Soil

Development in Austin, Texas

Rob Forkner





Images from USDA


Shrink/Swell factor in Austin Soils

Shrink/Swell in soil is a function

of seasonality (wet/dry) and

amount of illitic clay within the soil


Map from STATSGO, TX statewide

Soil data.


How can we use spatial analysis to determine

a more quantitative relationship

between soils and bedrock?

  • Rasterize data so that spatial analyst can work with it.
  • Reclassify data into groups of similar erodability.
  • Assign weights to erodability groups:
  • higher weight = more erodable
  • 4) Add raster layers to determine combined erodability.

Method Developed by Dr. Mark Helper, UT DOGS


Rasterized geological map with geological units grouped into formations.

Reclassified geological map with

formations classified into erodability

groups. More resistant rocks have

lower erodability. Limestone=1, Marl=2



Slopes on Austin West DEM

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


Annual precip for Austin, 2001. Map from NRCS/USDA

Brown area gets 1-1.5 inches more precip. per year.

For erosion weighting, Blue area = 1, brown area = 2


The Combination…

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

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


what s next
What’s Next?
  • Add a erodability parameter for vegetation cover and soil
  • Be more quantifiable with parameters for rock