SURFICIAL MAPPING OF PICTURED ROCKS NATIONAL LAKESHORE, MICHIGAN: INDIAN TOWN 7.5-MINUTE QUADRANGLE

Pictured Rocks National Lakeshore (PIRO) is a 43-mile stretch of coastline along the southern shore of Lake Superior in Michigan’s Upper Peninsula. Although most famous for its shoreline exposures of Paleozoic sandstone cliffs, Quaternary sediments from the most recent glaciation blanket the bedrock in striking patterns, highly contributing to the park’s appeal. Despite its popularity, PIRO remains the only U.S. National Park without a detailed surficial map.

Characterization of the surficial sediments and landforms is essential for accurately determining the geomorphic history responsible for creating this scenic landscape. It was proposed to construct 1:24000 scale surficial geologic maps of each of the eleven 7.5-minute quadrangles (quads) that encompass the park boundary in an effort to interpret the glacial history of this region. Mapping began in the western portion of PIRO and will progress to the east until all eleven quads are thoroughly investigated. The Indian Town quad was prioritized for this paper due to the previous lack of Quaternary study in the western portion of the park.

The most prominent topographic feature observed in the Indian Town quad is Miners River valley. This north-south trending linear feature is interpreted as a subglacial tunnel valley, likely formed from pressurized meltwater near the ice margin. The southwest portion of the quad is characterized by a hummocky topography consisting of very sandy material. This is interpreted as supraglacial outwash that was deposited over blocks of stagnant ice, collapsing into the uneven terrain following melting. The southeast section shows a subtle ice marginal position, delineated by a distinct change in topographic pattern. The crescent-shaped ridge in the central-western portion shows inclined, southeasterly plunging folds and dipping beds within the sandy material, which suggests glaciotectonic influence in this area.

It was quickly recognized that the glacial material in PIRO is predominantly composed of sand (e.g. sandy outwash, sandy till). Areas where bedrock is at or near the surface also yields sandy material as partially weathered sandstone. This creates challenges when classifying the sediment; however, extensive field observations have allowed the production of a detailed surficial geologic map of the Indian Town quad.