Content adapted with permission from the Indiana Geological Survey, a partner in the Natural Heritage of Indiana project. Credit for this article goes to Ned Bleuer and Deborah DeChurch. More information can be found here.
Native Americans called it Mikesen – the large lake. Its basin was scoured out by glaciers, and soon filled with their melt water. In time, it became the southernmost point of the largest system of freshwater on earth, the Great Lakes.
The movement of glaciers across Indiana dramatically altered the landscape. Although less dramatic, modern geologic processes continue to shape the land. Three of the more active types of processes are earthquakes in the lower Wabash and Ohio River valleys, stream and river erosion, deposition, and meandering throughout the state, and shoreline erosion and deposition along the Indiana's coast with Lake Michigan. Commonly these processes are termed "geologic hazards." But as we will see for the northwestern part of the state, what we call hazards today are processes that have acted on the shoreline for many thousands of years.
About 14,000 years ago, the glacier that occupied Lake Michigan (Lake Michigan lobe) began to pull back from a large arcuate highland that flanked the southern part of the lake basin. This highland consists of glacial moraines that mark the positions to which the ice advanced to and retreated over several thousand years. The last pull-back from the highland area created a lake between the retreating ice and the moraines. Essentially, the moraines acted as a dam at the southern tip of Lake Michigan that water could pond behind. Waves that lapped up against the moraines formed the first Lake Michigan shoreline, in what is now called the Glenwood Phase of ancestral Lake Michigan, eight miles landward of the modern shore. This shoreline is called the Glenwood Beach (see map below). In parts, this "fossil" beach is only a little wave-washed pebbles and some dune sand, but in other areas the Glenwood Beach is a large multi-fingered sandspit capped by dunes.
About 5,500 years ago, the level of the water surface in Lake Michigan was about 23 feet higher than today. At that time, the southern shore of Lake Michigan was in the early stages of forming the last of Indiana's shorelines—the Toleston Beach. In fact it is still in the process of making the Toleston Beach, today. Most people think of the Toleston Beach as the "Indiana Dunes." The Indiana Dunes, however, is only the area of the coast where there are high dunes. This area primarily runs from east of Michigan City westward to a little past Ogden Dunes (see middle figure on previous page). West and south of Ogden Dunes, the Toleston Beach fans out into more than 100 individual beach ridges. These ridges mark the position of Lake Michigan as sand was added to the shoreline. They can be thought of as rings in a bathtub or glass of Guiness stout as the liquid falls. Each mark, therefore, is younger than the previous.
A pattern of spit growth and beach-ridge development continued along the shore until man began to modify sediment transport patterns in the late 1800s and early 1900s. By the late 1800s the mouth of the Grand Calumet River had been driven more than nine miles (15 km) eastward across the Indiana shore to exit at Marquette, and the entire Toleston Beach shoreline had grown into the lake as much as 6 miles (9.5 km).
Today, Indiana's coast is divided into individual cells of shoreline between man-made structures. Patterns of sediment transport that occurred along the shore starting about 6,000 years ago are interrupted by these structures. However, waves that attacked the shore thousands of years ago have modern counterparts that are still trying to move sediment along Indiana's coast. These waves and associated currents cause erosion downdrift of man-made structures because sediment that would have nourished the shore from updrift of the structure cannot get around it. In a sense, the waves, currents, and lake-level fluctuations that shaped Indiana's coast are today a geologic hazard. More»