Fox Along the North Shore: Geological History of the Waterfalls
DULUTH, Minn — Along the North Shore of Lake Superior, there are numerous waterfalls, each with their own beauty and awe. Despite their unique appearance, they share a common history in how they were formed starting over a billion years ago.
“So, we had about 1.1 billion years ago there was a major event. It’s now known as the mid-continent rift. Essentially, the land that is now Wisconsin started to pull away from Minnesota. Volcanoes opened up and they started to have lots and lots of lava flows,” said Karen Gran, Earth and Environmental Sciences Professor at UMD.
These lava flows would go on to create a layered rock foundation. In some places it is up to 10 miles thick. That is much of what we see around here today. Professor Gran tells us these lava flows were only the first step in the formation of the North Shore.
“We like to talk about the history of the North Shore as being born of fire and ice. Eventually we get to the present, the last few million years, when glaciers came through here. And so, the two events are related. So, when the glaciers came through this area, they basically went right down the axis of the mid-continent rift and carved out Lake Superior,” said Gran.
Glacial melt led to the formation of many rivers that would go on to fill up what we now know as Lake Superior. Over time, waterfalls would form along these rivers.
“In order to get waterfalls, you need to have two different things. You need to have some kind of competent rock. And often you have, like stacked stratigraphy, so different rocks of different strengths stacked on top of each other. And that way the river can erode back on the weaker rocks, and then form these steep waterfalls on the more competent rocks. So that’s one thing, you need the stacked layers of rocks with different strengths. But the other is you actually need some relief. You need a lot of elevation drop over a relatively short distance,” said Gran.
Because of how Lake Superior was carved out by the glaciers, water has a very short distance to drop. Along the North Shore, water starts at over 1300 feet above sea level. It then flows down to Lake Superior which sits at 600 feet above sea level. A total drop of about 700 ft.
“If we had, you know, a hundred miles to get from the top of the hill out to the lake, we have no need of waterfalls. But instead, we have like, maybe two miles. And so, the river has to descend very quickly. And instead of it just being, you know, all the same slope all the way down, you find that it’s steeper at these really solid interiors of the lava flows, and then it will go for a while at a lower gradient. And then it will cut through the next lava flow and then go for a while and cut on down,” said Gran.
Over time, the landscape will continue to evolve as long as the water continues to flow, especially like it is now during the spring melt. Once the spring melt comes to an end, these waterfalls will rely on rainstorms and current ground water to keep the majesty flowing.
