Indiana Dunes National Lakeshore - Geology of The Lakeshore

Geology of The Lakeshore

The Lake Michigan Basin was formed during the Wisconsin Glacial Period. The Michigan Lobe of the continental glacier began its retreat northward over 20,000 years before present (YBP) forming the southern shore of the Lake Michigan Basin.

The Valparaiso Moraine is the dominant geologic form that created the various landscape forms of the Indiana Dunes, about 40,000 YBP. Within the arc created by the Valparaiso Moraine are two younger recessional moraines of the Tinley Moraine and the Lake Border Moraine. Each moraine created an artificial dam across the southern outflow of the melt waters of the receding glaciers. As each glacial lake breached a low spot in the moraines, water levels receded, leaving a series of shorelines and dune ridges.

The Calumet Shoreline is the oldest visible shoreline of Lake Michigan. It is a visible a sand ridge along Ridge Road through Lake and Porter Counties, Indiana. Two older shorelines, the Tolleston and the Glenwood Shoreline are much harder to identify and further south in the counties.

During the periods of glacial retreat, there were periods of stability. During these times, glacial lakes formed along the southern borders of the glaciers, bound into the Lake Michigan Basin by the recessional moraines. Four major glacial lake periods created the Indiana Dunes. They are the glacial Lake Chicago (14,000 YBP), Glacial Lake Algonquin (9,000 YBP), glacial Lake Chippewa (7,000 YBP), and Lake Nipissing stage (4,000 YBP). Once the glaciers had fully retreated from the Lake Michigan basin, post Lake Nipissing stage, the same factors that created the dunes south of the current shoreline, expanded the existing shoreline. The littoral currents or Longshore drift transport sand southward along the eastern shore of Lake Michigan. As they encounter streams bringing water from inland, sandbars are created, pointing down current, either southward if the shore is north to south or westward along the southern shore itself. If the currents are strong enough as they were when the Glenwood Shoreline was created, shallow inland bays would be formed with a wide sand spit between it and the open lake. Over time, the sand spits would merge with the far shoreline forming interdunal ponds. Each sand spit would be come a dune ridge. As the ponds filled in and wind built the outer sand ridges higher, the ponds would dry up and only a stream would remain, as the Little Calumet River does today, just south the state and national parks. As the shoreline moved northward, new ridges formed, additional streams, now slower and less powerful formed and the process duplicated itself. To the west of the Indiana Dunes, Wolf Lake in Hammond, Indiana forms a western border to the dunes. Here the same process is at work, only the littoral drift is again south, but along the western shore, pushing the sand and sand spits eastward. Today, it is the remants of the marsh lands and inter-dunal or inter-sand spit lakes that have formed this region over 40,000 years.