Tuesday, October 27, 2020

Geology of the National Parks in Pictures - Indiana Dunes National Park

 My next post about the Geology of the National Parks Through Pictures is a park we visited back in 2008 on our move out to Utah from New York. It was actually the only park we hit up on the trip out.

You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.


We had initially visited this park when it was still Indiana Dunes National Seashore. And since we had the dogs with us while we were moving across country we were limited on where we could go. But this seemed like a nice park to let them get out and run around for a bit. At the time I wasn't also taking geologically themed photos, so these are what I have. 

Walking down to the lakeshore. Indiana Dunes sits at the southern extent of Lake Michigan, the third largest Great Lake, and the sixth largest lake in the world. Lake Michigan and the surrounding landscape was a product of glaciation. Glaciers are like giant bulldozers made of ice and snow. During the last Ice Age, over 10,000 years ago, glaciers had come through and plowed out the basin in which Lake Michigan now sits. Eventually the weather got too warm for the glaciers and they started to melt. At that point, the glaciers start acting like conveyor belts, transporting rocks, soil, sand, boulders, and anything else that gets in their way to the end point of the glacier. Here all that debris gets dumped into one giant pile known as a moraine. The material within the pile is called till and is composed of all the glacial debris with a large amount of clay produced by the grinding action of the glacier. Towards the end of the Ice Age, as the glaciers started to retreat, they formed a moraine near the southern tip of Lake Michigan called the Valparaiso Moraine. Before there was Lake Michigan though, there was another lake. Lake Michigan formed from glacial Lake Chicago, a lake roughly the same shape as Lake Michigan but the water level was much higher. Lake Chicago formed about 12,000 years ago and was dammed to the south by the Valparaiso Moraine along the northern edge of Indiana. 

As Lake Chicago slowly dwindled down to the present day Lake Michigan about 2,000 years ago, several lakeshore benches were cut into the moraine at different elevations. The present lake level is at 580 feet above sea level, while there are lakeshore terraces located at 605, 620, and 640 feet above sea level. Much of the sand that forms Indiana Dunes comes from reworking of the Valparaiso Moraine. Walking towards the shore you come across this large flat area before descending abruptly to the beach. This large flat area is the last lake level of Lake Chicago before it lowered to the present day Lake Michigan levels. This is the 605 feet above sea level beach terrace known as the Tolleston stage. The Tolleston stage of Lake Chicago lasted from 8,000 to about 2,000 years ago. 

We are getting excited for our beach trip, looking out from the edge of the Tolleston stage terrace.

On the slope down from the Tolleston stage terrace to the modern day Lake Michigan. The sand dunes built up over time along all the stages of Lake Chicago as well as the modern day Lake Michigan. The sand, much of it reworked glacial deposits, are moved about the shore by a couple of processes. Winds are able to pick up the sand and blow them across the beach surface to accumulate behind the beach, also known as the foredunes. The foredunes are just above high tide levels of the water that would erode away much of the sand accumulation. Longshore currents also move sand along the beach. The longshore currents are currents that move parallel to the shore. These currents travel east to west and carry sand deposited from up-current river deltas and replenish any sand lost during the year from storm activities or other natural erosional events. 

One of the major features of dunes that prevents them from continually being destroyed during storms and high winds are the grasses that grow on them. These fast growing grasses act as anchors as well as wind baffles that end up not only preventing sand from being blow away but actively promote dune growth. The baffle slows the wind down, causing whatever sand the wind is carrying to drop out. 

Here, with a view along the lakeshore itself, you can see the foreshore dunes off to the left with their rather significant slope up to the lake terraces. You can also see the disparate sizes of the sediment along the foreshore of the beach. Here the larger sized sediment is located along the water-beach interface, This is because the finer sand size particles are either blown further up-beach or carried back out into the lake by larger waves. 

You can see the rather course sediment along the shoreline pretty well here. Much of this coarser sediment would have been brought here with the glaciers from further up north, mostly from Canada. Then the sediment would have been reworked by stream and lake water erosion over the last 12,000 years.


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