Geologic Time & Ancient Environments > After the Thaw

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Making Beach

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While the Toleston Beach from Ogden Dunes to Michigan City was making dunes and the Great Marsh was changing from marl ponds to a widespread peatland, the Toleston Beach west of Ogden Dunes was making new lakefront property. This lakefront property was in the form of a barrier beach of sand that extended westward into Illinois (see first image in diagram to the right). At the western end was an opening (Sag channel) that let water from ancestral Lake Michigan flow into the Chicago River. Landward of the Toleston Beach was a shallow lagoon (Calumet lagoon) that also drained westward into the Sag opening. This lagoon occupied the low area between the early Toleston Beach and the landward Calumet Beach. Internally, the western part of the early Toleston Beach consisted of long linear sand ridges that paralleled the shoreline. Geologists call these deposits beach ridges, and they mark successive intervals of shoreline addition. In fact, they are created by fluctuations in lake level with each ridge indicating a rise and fall of lake level during a roughly 30-year period of time.

By about 2,200 years ago, lake level had fallen and water could no longer exit through the Sag opening (see second image in the diagram above). Also at this time, the Calumet lagoon had shrunk in size to become the westward-draining Little Calumet River. The Little Calumet turned around at the Sag opening and drained back eastward into Lake Michigan in what is now called the Grand Calumet River. Throughout the central part of the Toleston Beach, beach ridges continued to form, adding 100 to 200 more feet of lakeshore with the creation of each ridge. Westward, however, sand spits developed off of a bedrock headland in Illinois. These spits began to extend southward toward Indiana's coast.

Aerial photograph (1938) of beach ridges in the Toleston Beach, just west of Gary. Each of the light-colored southeast- to northwest-oriented arcs is a former position of Lake Michigan's shoreline. The dark areas between ridges are wetlands.

Lake level was high in Lake Michigan about 1,700 years ago. This high stage caused erosion of the east and west margins of the lake. By 1,500 years ago, the sediment eroded from the western margin of the lake had formed a sand spit that extended off of the bedrock highland in Illinois and reattached farther south in Indiana (see third image in the diagram above). This spit trapped part of Lake Michigan behind it, creating early Lake Calumet. Most of the central part of Indiana's shore accepted the eroded sediment, and it was incorporated into more beach ridges. The addition of these beach ridges successively forced the mouth of the Grand Calumet River farther to the east.

This 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

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.

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