Geology of the Study Area
The Lake Superior basin formed from geologic faulting before glaciation of the Great Lakes Basin; however, the advances and retreats of the Laurentide Ice Sheet during the late Wisconsin glaciation 25,000 to 10,000 years ago carved and scoured Lake Superior basin into its present form (Mickelson et al. 2004). The cohesive soils found on the Wisconsin shoreline are a stark contrast with the bedrock of the north shore in Minnesota (Johnson and Johnston, 1995).
The main bluff materials in the study area are glacial deposits. Three glacial till units of in the study area are, from oldest to youngest, the Jardine Creek (Till) member of the Copper Falls Formation and the Hanson Creek (Till) and Douglas (Till) members of the Miller Creek Formation. The Jardine Creek Till is a sandy clay, and the Hanson Creek and Douglas Tills are fat clays. Other materials of importance in the study area are a red Precambrian sandstone and extensive deposits of glacial outwash.
The extent of the bluff materials is spatially variable, but the within a bluff profile the stratigraphy is generally simple. The Douglas Till is the surface unit throughout most of the study area and two facies, clay and clay loam, are present in the bluffs. The Hanson Creek Till is found at the base of the bluffs west of the Iron River and is overlain by the Douglas clay facies. The Jardine Creek Till is generally present over the sandstone bedrock, which was observed to outcrop discontinuously along the shoreline east of Quarry Point. Extensive sand and gravel outwash deposits are the main bluff forming materials between Port Wing and Herbster and also between Herbster and Bark Point.
Beach materials ranged from fine sand to cobbles and boulders. Site beach materials are characterized by three categories: sand, mix, and cobble. Sand beaches are those composed of ~75% or greater sand. Mix beaches are those ranging between ~75% to ~25% sand, and the remaining material is cobbles, boulders, and gravel. Cobble beaches are composed of ~75% or greater cobbles, boulders, and gravel. These characterizations are by percent volume, and beaches were characterized visually. Another beach material, found most often on wide sand beaches but seen among all beach types, is driftwood. Accumulations of driftwood may be highly temporal but can significantly affect beach morphology by creating steps and barriers which hinder sand migration and promote beach buildup and vegetation.