Glacial Geology

Glaciers covered most of the Great Lakes basin about 14,000 years ago (Figure 1). They began receding approximately 10,000 years ago (Figure 2). There were numerous minor advances of the ice edge. During each of these advances clay, silt, sand and rocks were left behind as layers of till which are now exposed in eroding bluffs in many places.

Figure 1 - 14,000 Years Ago

Figure 2 - 9,000 Years Ago

Between these till layers are layers or lenses of sand and gravel (Figure 3). Many of these layers were deposited as beaches and stream deltas at the margins of glacier and lake. Water drains through these porous layers, creating bluff instability problems.

The varieties of soil types are particularly noticeable in high coastal bluffs. Some soils like clay (Figure 4) can stand as very steep slopes when dry, but then fail as large landslides when wet or severely undercut.

Figure 3 - Layers of till

Figure 4 - Clay Bluff

Sand holds a more gentle slope and rarely fails catastrophically (Figure 5). In some places the shoreline consists of rock, with little or no sediment cover. This is especially true in the northern Great Lakes area where the glacier was mostly erosive, and the rock was resistant enough to withstand glacial erosion (Figure 6).

Figure 5 - Sandy Shoreline

Figure 6 - Rocky Shoreline

The present shoreline position is not the shoreline position of the past. In bluff areas the shoreline may have retreated several miles since the last glacier melted away. Even bedrock shorelines have been eroded by waves, though to a lesser extent.

Coastal property owners who plan to own the property for a long time are advised to be prepared for future lake levels beyond the ranges indicated in the historical records (see Sources of Information for the location of these ranges). The Great Lakes have been in their present connected arrangement for the past 3,000-4,000 years. The water level fluctuations over this time were the result of natural climate variability occurring from season-to-season, year-to-year, and from decade-to-decade. The record of measured lake levels (more than 140 years) is inadequate to confidently predict lake levels that will occur in the next 20, 50 or 100 years.