GEOLOGIC MAPPING OF QUATERNARY GLACIAL AND LACUSTRINE SEDIMENTS ON BEAVER ISLAND, MICHIGAN

Through a grant from the USGS, this project created a Quaternary geologic map of Beaver Island at the 1:24000 scale. Mapping was completed using analysis of aerial photography, gravel pits, and hand-dug test pits distributed across the island. Approximately 9 weeks were spent during the summer of 2004 making observations in person. This time allowed for detailed field mapping with much data being extracted from the gravel pits where stratigraphic cross sections were measured and described. Approximately 90 test pits ranging from 3 to 9 feet in depth were dug across the island in order to sample and describe the material. These test pits as well as observations of the terrain allowed for the creation of a detailed map of the units and geomorphology of the island. The map includes five main depositional units, as well as five geomorphologic landforms that were identified on the island.

The depositional units of the island include Glacial Till, Washed Glacial Till, Lacustrine Sands and Gravels, Dune Sand, and Peat and Muck. The glacial till of the island is very similar to the Kalkaska till of Northern Michigan in that it is a red clay-based till with poorly sorted rock fragments. This till, when washed, is generally discolored from organic and mineral-rich waters, and frequently has a clay and/or hardened sand layer above it. The lacustrine deposits are well sorted sands and gravels showing distinct bedding. Both the till and the lacustrine material were found to contain local limestone and dolostone, as well as transported sediments. The dune sand on the island is very well sorted silica sand showing well preserved crossbedding. Peat and muck prevented sampling in many places on the island, and prohibited to identification of the deposits beneath.

Geomorphologic units on the island included scarps, wave-cut terraces, beach ridges, paleo-wind direction as determined from stable dunes, and glacial fluted landforms. The most obvious landforms on the island are a system of scarps and wave-cut terraces which show the approximate location of the shorelines of the paleo-lakes which surrounded the island. These features are strongest in the west, but an intricate pattern of beach ridges can be seen covering the entire island, and is a good indicator of the path taken by the receding shorelines, as well as the shape of the old islands.