Study Area

GImapColor

Grand Island, near the town of Munising, Michigan, is the largest island on the southern shore of Lake Superior. The island is part of the Hiawatha National Forest, limiting present and future development to hiking and biking trails. Cliffs of Jacobsville Formation and the Chapel Rock Member of the Munising Formation sandstone rim Grand Island. A 2km long, 0.75km wide sandy tombolo connects eastern and western bedrock lobes. Sand is likely supplied from the erosion of nearby sandstone and glacial till. It is believed that wave refraction around bedrock lobes helps to concentrate and deposit sediment on the Grand Island Tombolo. Several lake level phases have been mapped through the identification of relic shorelines (Farrand and Drexler, 1984) and strandplain sequences (Johnston et. al, 2012). A complex internal architecture,  recording the evolution of the Grand Island Tombolo, has developed as a result of dynamic lake level fluctuations.

Sediment inputs cause coastal landforms prograde (build out from shore) (Ritter et al., 2005). Though progradation during regression (relative sea level fall) is widely accepted, how, or even if, progradation during transgressive (relative sea level rise) occurs is not well understood (McCubbin, 1982). Coastal landform morphology varies due to differing wave and tidal regimes as well as coastline geography. Notable coastal landforms include cuspate forelands, tombolos and strandplains. Cuspate forelands are triangular forelands t­­hat prograde offshore. Tombolos consist of a sand or gravel spit connecting two pieces of land and forms through wave refraction around an island, generally prograding in two directions. Strandplains are a specific kind of beach deposit forming from an interaction of wave deposition and windblown dunes creating a sequence of shore parallel beachridges (Ritter et al., 2005) and can be used to infer historic sea levels (Johnston et al., 2012).

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