Discovering the Bedrock and Glacial Geologic History of
Pymatuning Lake State Park, Ashtabula County, Ohio
Samuel B. C. Timko, [email protected], Nicholas W. Amerin, [email protected], and Dr. David B. Hacker
Department of Geology
Kent State University, Kent, Ohio 44242
History and
Features
Abstract
Pymatuning Lake State Park is located on the
wetlands surrounding the Shenango River in Ashtabula
County, Ohio and Crawford County, Pennsylvania. In 1933,
the Pymatuning Lake Reservoir was completed on the
river for flood control, water supply, recreation, and fish
and wildlife management. In addition to the dam, the
Espyville-Andover Highway was completed to cross the
lake. However, detailed geologic information of the park is
lacking for the public to understand its ancient history. A
detailed investigation was conducted to better constrain
the geologic history of Pymatuning Lake State Park.
Detailed information of the park was gathered
through extensive fieldwork and published data. Bedrock
and glacial geologic maps were constructed this data.
Thickness of glacial material and bedrock formations were
determined using ground water well data. A cross section
through the park was produced, showing the geology in
the third dimension.
Pymatuning Lake State Park is situated on the
glaciated plateau of northeastern Ohio and northwestern
Pennsylvania border where Pleistocene glaciers advanced
over the gentle hills and stream valleys of the land formerly
uplifted as part of the Appalachian Mountain building
process. The subsurface bedrock units in the park consist
of Devonian shales. The bedrock was covered by several
ice advances of the Wisconsinan Glacier that left behind
glacial features consisting of the Hiram, Lavery, and Kent
ground moraines, and the Defiance end moraine north of
the park. These glacial units completely cover the bedrock,
leaving minimum exposures in the area.
The geologic features of Pymatuning State Park
exhibit classic features of horizontal bedrock deposition
and glacial topography. Bedrock in the park was deposited
during the Devonian period (416-359 mya) of the Paleozoic
Era. However, any younger rocks that may have been
deposited after this time period were removed by stream
erosion, leaving behind a relatively shallow stream valley.
The arrival of the Wisconsinan Glacier (24,000 ybp) during
the Pleistocene Epoch (2.5 mya 11,000 ybp), blanketed the
area with glacial sediments, leaving little to no bedrock
exposures. Pymatuning sits atop a partially filled shallow
stream valley, ranging from 10 to 100 feet deep in some
areas, filled with glacial sediments.
Hydrogeology and Hydrology
Groundwater is the water found beneath the surface in the pore spaces and fractures of
bedrock or glacial till. Aquifers are connected units of permeable rock or till through which
groundwater can flow. Generally, the larger the grain size, the better the transmissivity and
storativity of a unit. Therefore, shales and other clayey units generally have low transmissivity,
and average storativity.
The Pymatuning area features groundwater in both its glacial till and shale bedrock units.
The Devonian bedrock units typically yield 1-3 gpm (gallons per minute), and the water quality is
low. The glacial till layer performs somewhat better depending on the thickness and location, but
due to its clayey and silty grain size, it may perform poorly at those areas. The glacial till in the
buried stream valley supplies the largest amount of usable groundwater.
Recharge of groundwater in this area is primarily through precipitation, though there may
be difficulty in areas of thick shale or clay.
Bedrock Geology
The bedrock layers of Pymatuning State Park consist entirely of Devonian shales
and sandstone. Shale is a fine-grained sedimentary rock composed of mud, clay minerals
such as mica, and silt to clay sized particles. Sandstone is a coarse-grained sedimentary
rock composed of fine to coarse sand sized particles, and minerals such as quartz and
feldspar. Both are typically deposited in shallow marine environments or transitional
environments (beaches, deltas, etc.), though shale can also be deposited in deeper
offshore waters.
The Conewago Group is the name given to the bedrock units beneath the park. It is
composed of shale, with some interbedded units of sandstone or siltstone. It is covered by
glacial till, and ranges from the surface to 90-110 feet subsurface. Below this shale layer is
a sandstone unit. This is the Le-Boeuf Sandstone Member of the Cattaraugus formation.
These formations are mostly horizontally bedded, and feature little deformation.
Other bedrock units in the general area, but not a part of the park, include the
Cussewago sandstone southeast of the lake. This unit is Mississippian in age.
Wentworth Grain Size Chart derived from published data on pubs.usgs.gov
Photo cred: Pennsylvania Geological Survey
Glacial Geology
Glacial till is the general term for unsorted sediment that is directly deposited by the
advancing or melting of a glacier. It is made up of sediments previously eroded by the glacier.
Pymatuning features both ground moraines and end moraines, two different types of depositional
features. Till thickness ranges in 5 feet in the hills to up to 100 feet in stream valleys.
The major glacial units consist of the Hiram Till, made up of the Hiram ground moraine
(directly south of the park) and Defiance end moraine (north and west of the park), depending on
the publication, the Lavery or Kent ground moraine (south, east and west of the park), and fluvial
outwash sediments carried by rivers. All of these units were deposited during the Wisconsinan
glaciation, the last glacier to cover this area during the Pleistocene.
Ground moraines are areas of hilly topography, notable for a lack of sharp ridges, and form
at the base of glaciers. End moraines typically feature ridges and layers of sediment that were
deposited at the edge of glacial advancement, and are used to mark the boundaries of glacial ice.
The Hiram till consists of silty and clayey sized particles. The Lavery/Kent till consists of silty
sized particles. The fluvial deposits consist of clay to gravel sized particles.
COMMONWEALTH OF PENNSYLVANIA
DEPARTMENT OF ENVIRONMENTAL RESOURCES
BUREAU OF
TOPOGRAPHIC AND GEOLOGIC SURVEY
Acknowledgements:
Ohio Geological
Survey
Pennsylvania
Geological Survey
Ohio Department
of Natural Resources
Pennsylvania Department
of Natural Resources
United States
Geologic Survey
Kent State University
Map Library