The major types of rock you will find in Duluth are:

Basalt Basalt is formed when lava rich in iron and magnesium erupts onto the earth's surface, cooling into a fine grained dark colored rock. It is the extrusive equivalent to gabbro.

Gabbro Gabbro is formed when molten rock (magma) is trapped beneath the land surface and cools slowly to form a coarsely crystalline rock (slow cooling gives crystals time to grow). It is the intrusive equivalent to basalt.

Diabase Diabase has crystals smaller than those of gabbro, and coarser than those of basalt, with the same chemical composition. It is formed when magma is forced into small underground cracks and cavities, cooling to create medium grained dikes and sills.

Sandstone Sandstone is the result of loose mineral grains of sand having been deposited layer upon layer, compacted by the weight of overlying material and cemented together over millions of years to form a hard rock. The appearance of sandstone varies depending on what types of mineral grains it's composed.

The Geology of Duluth

Bedrock geology map of Duluth

How were these rocks formed? See the regional summary.

Visit the real thing!

Evidence of Duluth's exciting geological past can be found all over town in outcrops of rock (surface exposures of bare rock, not covered by soil or vegetation). Good places to find outcrops are along the stream corridors. Streams continually cut down through soil and vegetation (and rock too!), unearthing the bedrock for our viewing pleasure.

Below are a few suggestions of rocks you can see while exploring Duluth.

1. Upper Keweenawan sediments can be found in outcrops in the southwest End of Duluth. These cross-bedded sandstones are at Mission Creek.

These rocks started out as particles of mud and sand that were washed into a lake and settled to the bottom (the same process that creates the nice sandy bottom of Lake Superior at Park Point).

You may notice that some of the layers are red and some are green. Both colors result from the iron in the sediments. Red sediments are caused by ferric iron combining with oxygen, forming the red rust similar to that seen on many cars around town. Green sediments occur when the ferric iron changes to ferrous iron.

To visit these rocks, turn north off of Hwy 23 onto 131st Avenue West. Drive to the end of 131st Avenue, and continue about 1/4 mile past the gate to the hiking trail parking lot. These rocks are a 10 minute walk from the lot.

2. Outcrops of the Duluth Gabbro Complex can be visited to the west of Mesaba Ave Westward. Some places to look are along Skyline Parkway near Twin Ponds and Enger Tower, or along Knowlton Creek, Keene Creek, Kinsgbury Creek. This photograph (coming soon) is of another gabbro outcrop, named "Point of Rocks" by locals.

3. Lief Erickson Park is a wonderful place to have a picnic, skip some rocks, and to see some great Duluth geology! Exposures of basalt and cross bedded sandstone can be viewed from the lake shore.

4. Eastward from Mesaba Avenue are numerous outcrops of volcanic lava flows with intruded diabase sills and dikes. A few examples can be seen Chester Creek, Tischer Creek and Lester River.

5. Although the last glaciers in Minnesota melted about 11,000 years ago, local rocks hold evidence of their passing. Outcrops at the mouth of Lester River are smooth but covered with scratches (glacial striations). Heavy glaciers with rocks and other debris at their bases worked like sandpaper to smooth out the rocks here, while individual pieces of grit scratched the glacial striations that you see.