1. Site Suitability Analysis of Periglacial Involutions in Southwest Colorado
Rae Kinney
Department of Geosciences, Fort Lewis College
Methodology: Georeferencing
Pleistocene glacial extent in the San Juan Mountains data by
Atwood and Mather (1932) and modified by Gillam (1998) was
scanned and imported into ArcMap. In order to accurately
georeference the LGM extent, numerous points were required.
The Animas River, the cities of Silverton, Durango, and
Farmington, as well as the Colorado/ New Mexico border were all
used as points to place the image in the correct location. After the
image was correctly positioned, the edit toolbar was used to
create a shapefile of the glacial extent.
Methodology: ModelBuilder
ModelBuilder was used in numerous steps of this project to
create appropriate criterion and to perform the overall suitability
analysis. In order for the LGM shapefile created from the
georeferenced figure above to be analyzed, it needed to be
converted into a raster data set. The polygon to raster tool was
used to convert the image. After the LGM extent was rasterized,
it could be analyzed using different tools. In this ModelBuilder,
the Euclidian distance tool was used along with the reclassify
tool to determine the most ideal conditions in zones around the
LGM extent shown in the image to the right.
Introduction
During the lake Wisconsin glaciation, ice extended approximately 5,000
km2 in the San Juan Mountains, the largest glacier being the Animas
Glacier, which covered 600 km2 of the total area (Atwood and Mather,
1932). Beyond the glaciation coverage, periglacial processes such as
freezing and thawing of soil layers and perennially frozen ground. During
this time, freeze/thaw processes contributed to the formations of
involutions. An involution is a deformation of previously horizontal soil
layers into interpenetrating structures due to the expansion and reduction
of ground ice. The purpose of this suitability analysis was to determine
adequate locations for possible periglacial involution structures through
performing different analyses using ArcGIS.
Results/Conclusion
Each criteria used in this analysis was reclassified with values 1-10; 1 representing low suitability and
10 representing high suitability. The raster calculator was used to classify areas depending on the
weight that was given to each criteria. Aspect received 30% and was reclassified with north,
northeast, and northwest having values of 10. Slope was given 30% and reclassified in natural
breaks with lower angles receiving higher values. LGM Euclidean distance was given 20% and was
reclassified with areas farthest and closest to the glacier with lower values. Timberline received 20%
and was reclassified given areas above 2,400 meters a value of 10. The output was on a scale of 1-
10. The final map represents areas that received a value of 8 and higher.
References
Atwood, W., and Mather, K., 1932, Physiography and Quaternary Geology of the San Juan Mountains, Colorado: U.S. Geological Survey
Professional Paper, v. 166, p. 176.
Blagbrough, J.W. 1998. Late Wisconsin Climatic Inferences from Rock Glaciers in South-central and West-central New Mexico and East-central
Arizona: New Mexico Geology, v. 16, p. 65-71
Gillam, M. 1998. Late Cenozoic Geology and Soils of the Lower Animas River Valley, Colorado and New Mexico [Ph.D. thesis]: Department of
Geosciences, University of Colorado, p. 1-473.
Ray, N. and J. M. Adams. 2001. A GIS-based Vegetation Map of the World at the Last Glacial
Maximum (25,000-15,000 BP) Internet Archaeology v. 11 (http://intarch.ac.uk/journal/issue11/rayadams_toc.html)
Data Sources
DEM – USGS, LGM extent map – Mary Gillam (1998), LGM vegetation – Quaternary Environments Network
Creating the Site
Suitability Analysis
Polygon to Raster
Analysis
Aspect – An aspect surface analysis was
generated from downloaded and mosaicked
DEMs to display slope direction. North, northeast,
and northwest facing slopes receive less
exposure to sunlight, thus remaining colder. This
is important to the formation of involutions due to
the freeze/thaw process that is more likely to
occur on north, northeast, and northwest facing
slopes.
Timberline – Timberlines are accurate indicators
of current climates. Data from the Quaternary
Environments Network of LGM vegetation was
compared to a study from Blagbrough (1994) of
rock glacier formations in northern New Mexico to
determine an estimate of the timberline during the
LGM. The raster calculator was used to select for
elevations above 2,400 meters using downloaded
and mosaicked DEMs from the USGS. The
estimate of 2,400 meters used from Blagbrough’s
research was similar to the boundary between
polar and alpine desert and steppe-tundra from
Ray and Adam (2001). Timberline is an important
component in determining past environments
since it indicates at what elevations cold climates
existed.
Slope – Another surface analysis was performed
to determine the slope angles from mosaicked
DEMs. The slope tool was used to create a map
showing a range of gentle to steep slopes in
southwest Colorado. Gentler slopes are favored
for the preservation of periglacial features. A
steep slope has the potential to disrupt possible
involuted layers as well as contribute to other
processes that would distort the structures.