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
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
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
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.