My place is a stretch of beach along the southern tip of Lake Michigan. One question I have always had about the lake since I started going there as a little kid is how did Lake Michigan form? Growing up in LA, I wasn’t accustomed to seeing many local lakes, especially not any that were the size of Lake Michigan. At the time, I might have mistook it for the ocean, but its fresh water left questions about its unique features. Considering the question now, glaciation is the most likely hypothesis, and the great lakes were most likely created by melted ice that filled in the land that the glacier eroded. I was initially drawn to this hypothesis because of the location’s northern latitude and the lake’s fresh water, but our study of proglacial lakes created more questions and evidence to confirm. To answer this question, I would have to look at the sediment record of the lake basin. Here, I would expect to find alternating coarse and fine sediment layers that produce larves. The coarse sediment would have been produced by the movement of waves during the summer, and the fine sediment is the product of the frozen lake water’s lack of movement during the winter. Additionally, I would expect to find large dropstones in the sediment record. These dropstones are the result of glacial moraines and the calving of icebergs containing large sediments that fall to the lake floor as the iceberg melts.
These observations would show that Lake Michigan is a glacial lake. It does not, however, tell the full story of its conception. I was also intrigued by the incredible size and depth of Lake Michigan, and wondered why it is so much bigger than other glacial lakes we studied like Lake Hitchcock, for example. What I discovered through research is that the incredible depth of the great lakes is a result of tectonic plates. Over one billion years ago, two tectonic plates that were fused together along the area that is now the midwest split apart, creating a midcontinent rift. The rift system is over a thousand miles long, and it forms a rift valley at the great lakes. This valley created basins at the rift’s fault lines, which were quite deep. So, the extreme depth of Lake Michigan is a combination of the valley formed by tectonic plates as well as the glaciers’ ability to carve out the deep basins. Lake Michigan was formed from the last glacial period, which ended about 11,000 years ago, so there was a relatively long period of time in between the formation of the basins and the glaciation processes that filled the basins with water. Evidence of this tectonic process would be seen in the geologic record in the form of igneous rock intruded underneath many layers of sediment deposited from the glacial lake. These igneous rocks are most likely basalt from lava and granite because they are the result of upwelling molten rock from the lithosphere.
Another aspect of the landscape that has interested me is the formation of sand dunes that lie behind the seashore. Lake Michigan is well known for its sand dunes, and I spent many hours when I was younger playing in the tall dunegrass and rolling down the piles of sand. The theme of lakeshore composition leads me to consider how these sand dunes were formed. Unlike the other physical components of the shoreline, the formation of the sand dune is controlled by wind as well as water. The wet sand that is deposited along the coast dries, and this dry sand is blown by onshore winds until it accumulates. Observations to answer these questions would then be limited to the amount of wet sand deposited by the ocean and the prevalence of offshore winds. Dune erosion has become an issue in my place, so the rate at which the dunes are being eroded would be an important observation to determine the magnitude of their effect on the coastline.
The issues of glaciation, tectonic movement, and shore composition has changed my understanding of Lake Michigan drastically, to the point where I will be reminded of these processes every time I’m there. When I am swimming out into the deep water I will look down and think about the processes that occurred far below my feet to form the lake basin. I will think about the rift below and the igneous rock that cooled many years ago, but was once molten. I will also look up above the water and consider the tremendous height of the glacier that resided in this location prior to Lake Michigan’s formation. I will consider the incredible weight of the ice and how this contributed to the carving of the basin. In my aunt’s house on the lake, she has a three dimensional map carved out of wood that shows a rough estimate of the depths of lake Michigan. The next time I look at this art, I will understand why the different depths are there and why there are anomalies. For example, I will see the collection of islands that litter the northern section of Lake Michigan and understand how the glacial movement was uneven, which allowed for these protrusions in the basin.
The walk to the beach from Lakeshore Drive and into the water will take on a new meaning for me now that I better understand the processes involved in the formation of each component of the area. I will first understand that the road which runs along the cliff above the beach extends far out from the shore because the waves have repeatedly carved into the cliff forcing it to be pushed farther out. Walking down the stairs and over the dunes, I will remember the importance of the transfer of wet sand from the waves so that windblown sand can continually supply the dunes. Without these large masses of sand, the many houses that stretch out along Lakeshore Drive would be defenseless, and the erosion could destroy their homes. Next, I walk down the backshore and step carefully across the small rocks that mark the boundary between the backshore and the foreshore. The rocks are likely glacial till deposited from the ice that formed the lakes. The rocks are smooth because they have been worn down from the force of the waves, and they are randomly sorted. Past the rocks, there is a sudden drop in elevation as I fall down the beach face. The water level remains at my shoulders as I pass through the low tide terrace until I am up a hill to the top of a sandbar. Looking out towards the shore, I understand how the movement of waves were able to form the topographic aspects of the mini journey I just took. I have a newfound respect and understanding for the ability of waves to slowly transform landscapes over thousands of years through the same repetitive actions. Recently, houses have been built up along the shore that extend out past the sand dunes and onto the backshore to maximize beach access. These houses have begun to sink into the sand and erode parts of the dune, ruining both the house and the beach. This is disappointing because it shows a lack of respect and understanding for the system of processes that control the landscape of the shore. By eroding the dune and trying to live on the shore, these houses have stopped the wind and wave processes that have been in control for thousands of years from being able to operate the beach.
As I walk back to the shore from the sandbar I notice that I am about 20 yards to the right of my beach chair and towel. I now understand that this is the longshore current pushing me parallel to the shore. During times of more severe weather, this current creates an undertow which can be extremely dangerous for swimmers. When I was younger, my uncle told me that if I felt the undertow pulling me away from the shore that I shouldn’t swim directly towards shore but rather parallel to it. I didn’t understand how this made any sense, but now I understand why this was smart advice. The swash of the waves moving away from the shore is so powerful during severe weather that it would drain any swimmer’s strength to try to work against it. Swimming parallel to the shore, I would be moving with the longshore drift and using much less energy. I have spent time in Lake Michigan throughout my entire life, so I have taken these processes for granted for a long time. Through geologic and scientific observations, I have gained a new perspective on this familiar place that has changed the way I feel about it forever.