The first time a professor began talking about the ways different lakes are formed I was fascinated at the complex history of these places I had been traveling to my whole life, yet, not once had I asked myself: “how did this take shape?”. I believe that day back in 2010 marked the start of my scientific fascination for lakes – and my career path. So simple, so present, yet stuff I never thought about. I’m just reminiscing as I sip my coffee with a window view of Lake Constance, thinking about how this particular lake and its surrounding smooth hills came to be what we see today.
How did it happen?
After the catastrophic events that brought the “Dinosaur-Era” to an end, a cooling down period began on Earth (65 million years ago (Ma)). Some million years later (30 Ma) glaciers began to form in Antarctica and, eventually (5 Ma), continental ice sheets (aka glaciers) formed, and what we know as the last Ice Age began.
I start with this to clear up a common misconception: the Earth did not freeze from the Poles down. Besides the ice sheets on the North and South Poles, most glaciers formed on top of mountains in North America, Europe, parts of Asia and South America (and small parts of Africa and Oceania). Remains of glaciers from the last Ice Age still exist in Greenland, Antarctica, and several glaciers from the Andes and the Alps, for example.
Glaciers are an accumulation of snow under the right conditions to turn to ice, forming layer over layer, allowing for massive ice formations. How massive, you ask? To paint you a picture, a glacier once covered the entire New York City even in altitude (I mean glaciers taller than the tallest existing skyscraper!). Under the right conditions, glaciers will keep getting bigger.
But glaciers are not static.
They don’t just sit there frozen and growing. Glaciers move. This massive ice blocks (a solid) move somewhat like a liquid over long periods of time. The pressure the glacier exerts on itself makes the lower parts melt, cracking and breaking the underlying terrain as the water freezes and thaws repeatedly. This thawing-freezing leads to the collection of debris and rocks which are then trapped and transported within the glacier. The liquid part allows the huge solid ice lump above to move along as if “swimming” on top of it.
As glaciers expand and move forward, they push the soft soil that lies over the hard bedrock. As this soil accumulates at the end of the glacier, they form structures called moraines, which you might commonly refer to as hills, ridges, or even mountains. You can think of these moraines as if the glacier had built its own dam.
Eventually, the Ice Age came to an end. With the warming period that began at the end of the last Ice Age, glaciers began to retreat. The basins they carved got exposed. With the help of some of the melted ice and later on precipitation, the basins filled up with water and these moraine-dammed lakes originated.
The Rhine Glacier
Lake Constance is one of these types of glacial lakes (moraine-dam). South from Lake Constance, you reach the Alps, where the Rhine glacier once dominated the landscape. The Rhine Glacier moved north from the Alps and carved this basin around 15,000 years ago.
After the weight of glaciers is removed, uplift of the Earth’s crust (glacial rebound) occurs. This, of course, also alters the terrain through tilting and other deformations.
Besides this glacial origin of Lake Constance, with time the transport of fine material by air (aeolian transport of loess) also brought in different layers of different materials to this basin, giving it it’s current pedological and even biological characteristics.
For more details of this image see Rades et al. (2016).
Most of North American and European lakes that we can appreciate today are young glacial lakes. Glaciers are amazing landscape architects, scooping and widening and polishing the landscape as they move…but they do a lot more than just moraine-dam lakes. We’ll talk about all other types of glacial lakes and structures another time, for now, let’s keep enjoying Lake Constance and its moraines.
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References
- Rades, E.F. et al. 2016. Luminescence dating of the Rissian type section in southern Germany as a base for correlation. Quaternary International.
- Müller, G. & R.A. Gees. 1968. Origin of the Lake Constance Basin. Nature. 217: 836-837.
- Wessels, M. 1998. Natural Environmental Changes Indicated by Late Glacial and Holocene sediments from Lake Constance, Germany. Palaeogeography, Palaeoclimatology, Palaeoecology. 1-4(140):421-432.