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Digging Duluth: Dirt Depths, Rocks, and Outcrops

Photo of a large outcropping called the Point of Rocks

Top of Point of Rocks Outcrop

The Duluth area's underground varies from point to point as much as the temperatures and the landscape. These sudden variations in Duluth's Weather and Duluth's topography are part of what gives Duluth its charm. As you drive around, and see the sudden changes from steep and hilly to flat and smooth, what's below is changing as well. Think of this when you view the hillside rising like a wall above the flat lake from the flat city of Superior.

Personally, I like these differences. After awhile in the hills, it feels good to go to wide open spaces where it is equally easy to walk or bike in either direction. It's just nice to see something different.

While the big differences in the two cities' steepness is easy to see, they differ in ways you can't see. When you have looked across those waters, have you considered that Duluth’s hillside is basically just one big rock? Yes, it puts on a nice layer of dirt and vegetation. But, scrape off that thin layer and the Duluth’s hill is just one big rock.

“That is true of anywhere, when you scrape through the glacial cover you get down to solid bedrock," said University of Minnesota Duluth Geology Professor Jim Miller. “Sometimes the bedrock is exposed through the cover and we call that outcrops.”

So, all these rocky cliffs, boulders, and masses of rock stuck into Duluth neighborhoods is the same bedrock, you find everywhere. Here, it is just really close to the surface.

“It is pretty thin on the hillside. Right along the edge, the dirt is probably no thicker than 20 or 30 feet,” said Miller. “There can be valleys in the bedrock where it is filled with sand and gravel. But, usually it is probably on the order of ten or 20 feet thick.”

In Superior, the layer of dirt is much thicker. The dirt is also thick above the hill.

"As you go up the hill, the glacial level gets progressively thicker," said Miller. “You run out of outcrop. You don't really see the bedrock. The dirt is getting on the order of hundreds of feet thick, when you are several miles inland from the lake.”

By the time you reach the airport, the bedrock is pretty deep and the landscape pretty Iowa-like. But, flat land does not always mean thick dirt. Take Lakeside Duluth.

Lakeside Duluth is pretty flat. Well, at least the area east of about the Northland Country Club is pretty flat. Here, outcroppings aren't that common. This might lead you to think the dirt is really thick here. When you reach the Amity and Lester, you then see the rock was always hiding, just a little below the surface.

Here the water is telling the true story. The currents have stripped away the thin layer of dirt and revealed the rocky skeleton below. But the water can’t wear down this rock as easily as the dirt. It has to crash many times on its way to the lake. It cannot enter the lake flowing flat and wide, like say the St. Louis River.

“When you get down in the lower parts, by the St. Louis estuary, then you have lots of fill," said Miller. "At one point, the river was about 300 feet lower than it is today. So, the St. Louis river was cutting down deeply into that valley to reach down to the low level of Lake Superior. Then, when the water started to rise, it backfilled that valley. If you drill a hole out in the middle of the St. Louis bay in the estuary, there you can go through hundreds of feet of sand and gravel and fill before you hit bedrock.”

In this area the dirt rules and the rock runs silent and runs deep. The flatness of Superior is also caused by this fill.

“Where highway 53 and 2 split off, you will notice a rise there. Then you are back to strong bedrock controlling the landscape. But everything pretty much there to the edge of Duluth is pretty much thick with fill,” said Miller.

At another of the area's famous flatlands, Park Point, the houses sit high above the bedrock. Sand drifting down from the St. Louis and Nemadji rivers collected long ago to form this long bar of sand, that so perfectly walls off the harbor, it almost seems man-made. Here along the shores there are no bedrock outcroppings, like there are at the competing shorelines of Brighton Beach and a hidden gem I like to call "Flat Rock Beach."

Even where the rock rules, there are enclaves of relative flatness. The type of rock in the bedrock can affect the height of the land in Duluth area.

“You can see some rocks are more resistant that others. So, the promontory above downtown is held up by this gabbro rock. Then downtown is underlined by these basalts which have a lot more fractures and they break more. So, they hold up low ground,” said Miller.

The rock below can give an area its shape, lack of shape, and various degrees of rockiness. This is caused by what Miller describes as “topography expressing what the underlying geology is.”