Geology of the Dunes Uncategorized

An interesting look at the Great Marsh and how it was created. – Pt. 4

January 14, 2016

Submitted by: Patty Kostro

As previously discussed in the December blog, the Lake Michigan shoreline, as we see it today, is due to many geologic changes over the past 14,000 years. As the Michigan glacial lobe advanced and retreated, Lake Michigan endured many different shapes and elevations due to the changing water levels and ice margins. These distinctive changes in the water levels or lake levels are called “phases”.  The different lake phases are important geologically because they are what began the creation of the different Lake Michigan shorelines or beaches.

There were five predominant phases. The first phase was the Glenwood followed by Two Creeks, Calumet, Chippewa and lastly, the Nipissing. The Nipissing phase is the phase that brought the present northwest Indiana shoreline, Toleston Beach. This is the third major shoreline for southern Lake Michigan and was preceded by the Glenwood and Calumet beaches. (you may recognize some of these names, because they are now trails that surround us! – be sure to check our trails & beaches section if you are interested in exploring the area!)

To understand the southern shore of Lake Michigan and its geologic processes the geomorphology should be taken into account. Geomorphology is simply a science that looks at how landforms are made. A major influence of the formation of landforms in northwest Indiana was due to glaciation. Some of these that were made by glaciation are the moraines and kettle lakes. Glacial landforms result from flowing ice or meltwater. However, we have coastal processes that have formed various landforms, too. These are made due to processes along a coast. Wind, waves, currents and sediment all play a role in the creation of a coastal setting.

We have a Great Marsh! (photo above was taken from Beverly Shores and is just a part of the Great Marsh, seen from atop Dune Ridge trail.)  Marshes are areas in lowlands that stay waterlogged and are filled with herbaceous plants rather than woody plants. A marsh was created between Calumet Beach and Toleston Beach east of Ogden Dunes to Michigan City. Remember, Calumet Beach existed prior to Toleston Beach. As the Toleston beach was being formed, it began as a barrier beach.

Great Marsh

A barrier beach is a sand bar that runs parallel with the shore and has a lagoon on the landward side. This type of sand bar is exposed during times of high water. The sand bar that was initially created started to migrate towards the shore. Powerful storms allowed strong wave action and water to lap over the barrier beach (sand bar) and deposited water, sand, gravel, mud and silt into a lagoon. This water and sediment was caught between the Toleston Beach sand bar and the beach of the previous Calumet Beach.  As lake levels stopped rising, the waves could not overlap the barrier bar anymore.  Eventually, the lagoon would begin to dry and a marsh would begin its formation. The Great Marsh is contained between Calumet Beach and the Toleston barrier beach.

Blog Toleston Beach Creation

But, there was another landform being made during this time: Sand dunes! Because, the lake level dropped, it exposed sand! And, that will be the topic of next month’s blog…

Did you know?

That Lake Michigan underwent isostatic rebound? The one to three mile thickness of glacial ice weighed down the area for thousands of years. As the ice melted, the crust of the earth would naturally want to “bounce” back or uplift to its natural position.  This may be a reason for changes in lake levels, too!

Toleston Beach or Tolleston Beach are technically referring to the same name. The word Toleston has been used in many scientific papers. They are interchangeable. Since, 2010 the spelling Tolleston is the acceptable way to be spelled going forward. However, for the purpose of the blog, you will still see the word Toleston being used.

You can read about more Geology of the Dunes right here!


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