After the Thaw

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Geologic Time & Ancient Environments

The Development of Lake Michigan

by Todd A. Thompson

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.

Glacial Retreat and Early Lake Michigan

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.

Distant Contact

Continued retreat of the Lake Michigan ice lobe from the Michigan basin opened outlets across Canada that rapidly lowered the elevation of the lake. The first time this happened was about 12,200 years ago. This time of low lake level is known as the Two Creeks Phase, and it ended about 11,800 years ago when ice readvanced into the basin. The readvance closed the Canadian outlets, ponding water in front of the ice once again. In the southern part of the lake, the rising lake level and accompanying waves and currents scoured sediment from the margins of the lake. This sediment was transported by longshore currents to northwestern Indiana. Here, it accumulated by waves and winds into the Calumet Beach (see map to the right). The Calumet Beach is a prominent feature along Indiana's coast. Highway 12 follows it from Michigan City to Dune Acres. Farther westward, Ridge Road follows the Calumet Beach through Glen Park, Highland, and Munster.

About 10,000 years ago, lake level rapidly fell again. It fell so quickly that nearshore bars are preserved in Wicker Park (intersection of Ridge Road and Highway 41). Like the fall 2,000 years earlier, this fall occurred as glacial ice retreated into Canada, opening an outlet across Canada into the St. Lawrence Seaway. Glacial ice would never return to Lake Michigan. This time of low lake level is know as the Chippewa Phase of ancestral Lake Michigan, and the elevation of the lake may have been more than 100 feet lower than today.

Moving in on Today

The Chippewa Phase lasted for about 4,000 years! Anyone standing on the shore of modern Lake Michigan could have looked northward and not seen water. Not only would they have not seen water, but it would have been more than a 5-mile walk for a drink.

The level of the water in the lake, however, was on the rise. By 8,300 years ago, it had drowned oak trees about 9 miles north of the modern coast. And by 6,300 years ago, the elevation of the lake was near its current position. The water continued to rise more than 20 feet higher than today by 5,500 years ago. The rising water was probably caused by two concurrent phenomena. One is a process called isostatic rebound. Isostatic rebound is an uplift of the earth's crust after a weight has been removed. In the Great Lakes area, the weight was the glacial ice. But now that the ice is gone, the crust is popping back up. In Hudson Bay, the rate of rebound is greater than an inch per year! The rebound more than 6,000 years ago elevated the outlets draining across Canada so that flow had to be redirected southward, following the current pattern of Great Lakes drainage.

The other reason for rising water levels is that the climate may have been cooling down from a warm and dry period (10,000 to 6,000 years ago) called the Hypsithermal. Think of it as the dust-bowl days on steroids -- hot and dry. Lake level in Lake Michigan is a mirror image of temperature. When temperature is high, there is less rainfall and more evaporation from the lake (see graphs to the right). Lake level falls. When it is cool, there is an increase in precipitation and a reduction in evaporation. Lake level rises. It is possible then that coming out of the Hypsithermal there was increased precipitation and less evaporation across the basin. This would have caused lake level to rise.

Whatever the actual cause, 5,500 years ago lake level stood 23 feet higher than today. Gary, East Chicago, the northern part of Hammond, Miller, Ogden Dunes, Dune Acres, Beverly Shores and the north part of Michigan City were under water. Of course they were not there then because the Toleston Beach had not yet been created. That, however, is the rest of northwestern Indiana's story.

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