Black Swamp Wildflowers dedicated to the restoration and preservation of the natural heritage of Northwest Ohio

Geological Analysis of the Site

The site of the Prairie Restoration has been explored by Graduate students from BGSU-Geology as part of their 2004/2005 graduate course in Engineering Geology. This summary is based on a report kindly provided by Ryan P. Murphy.

Geologic Setting

During the Precambrian volcanic activity and crustal rifting formed metamorphic rocks, which are only found at great depths (2,500-1,300 ft) today (ODNR, 2001).  In the Cambrian, Ordovician, Silurian, Devonian, Mississippian and Pennsylvanian Northwest Ohio went through numerous transgressions and regressions resulting in the deposition of marine deposits (ODNR, 2001).  Limestone and dolomite deposits in particular were left behind from shallow seas that existed during the Devonian. During the Permian and Tertiary, warm swamps were uplifted and eroded (ODNR, 2001).  During the Quaternary glaciers moved in and covered the northern parts of Ohio (ODNR, 2001). As they pushed forward, they picked up debris along the way, and deposited this fertile, mineral-rich glacial till on top
Prof. Charles M. Onasch, Chair of the Geology Department at BGSU, obtains soil samples using a trailer mounted auger.
of Devonian limestone and dolomite. As glaciers receded 20,000 years ago, a vast area of black, oozing swamp began filling the depressions that had formed under the glacier's weight.

Site Description

The topography for the area used in the geological analysis is flat with an elevation of approx. 200 meters (660 feet) above sea level. The area is adjacent to the western edge of the wooded section and is now part of the area where a tall-grass prairie is being restored. The highest point of the entire site is in the wooded area (201m) dropping to a low point at the north-eastern corner (199m).

Study Site. Darker shading indicates the area used for geological analysis.

Bedrock

Seismic data was analyzed to determine the type of bedrock present under the glacial till.  The velocities of the bedrock ranged from 3167 m/s to 4139 m/s.  These values are consistent for the velocity of limestone and/or dolomite.  Seismic and resistivety data were combined to get a better data set to analyze the depth of the bedrock.  These data were entered into GIS to contour the data (Figure 3).  These values range from 1.7m in the northern part of the site to 2.5 meters in the southern part of the site, resulting in a southern dip of the bedrock.  No structures were identified from the geophysical data.

Soils

Three soil layers were identified through soil sampling and resistivity methods, however the third is likely a transition into bedrock and the differences between layers 1 and 2 are according to resistively data is only in moisture content.  Layer one extends from the surface to layer 2 and is 0. 2 m thick.  Layer 2 extends from layer one to layer 3 and is approximately 0.4 m thick. The thickness of layer three, extending from layer 2 to bedrock, can not be determined from soils

Soil analysis. Plasticity index for the three layers of soil.

data. However the seismic data reveals depth to bedrock is on average approximately 2.0 m thick, therefore the layer 3 would be about 1.4 m thick. All three soils were classified as well graded sand. However there were slight variations in moisture content and optimum moisture content for the different soil layers (appendix 2).  The plastic limits for layers 1 and 2 were very similar; they averaged 21.7 and 19.7 respectively.  The liquid limits for layers 1 and 2 were also very similar; they averaged 32.8 and 36.8 respectively.  The plot of plastic index vs. liquid limit tells us the soils are “fat”.

Bedrock. Contoured Depth Data for the study site

Drainage

Field tiles drain a sections of the farm field. A collecting tile along the southern edge of the site carries the water west, it crosses Luckey Road, and empties into the ditch to the North Fork of the Portage River.

Literature cited

Links

http://www.emporia.edu/earthsci/student/damery1/gl_form.html


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