Geology Lesson #7, with Capt. Paul and Dr. Nat (also known as Paul and Nathalie Brandes). These two sure get around. Our Pasty Cam instructors take us to Marquette, Presque Isle for today’s lesson. Capt. Paul gives us a little teaser of background information and then Dr. Nat chimes in with a video explanation of this geologic feature.

“Attached are some photos of the Black Rocks area at Presque Isle, but the neat geologic feature is at the point. Below the parking area is a gap in time of about 1.7 billion years between the Black Rocks (peridotite) at 2.7 billion years old and the Jacobsville Sandstone at around 960 million. Just a little background:

Peridotite is a very dense, ultramafic igneous rock consisting of mostly olivine, pyroxene, and relatively high concentrations of magnesium, reflected in the large amounts of magnesium-rich olivine. Peridotite is mainly derived from Earth’s mantle and brought to the surface either by volcanic eruptions, or through uplift of older volcanic material. The peridotite at Presque Isle is not pure as it has been metamorphosed and today has a greenish cast owing to the addition of serpentine. In geologic terms, this rock type should be called serpentinized peridotite. Within the rock, veins of light colored dolomite and reddish colored chert can be found, as well as veins of pyrite, chalcopyrite, and galena near the northeast corner of the peninsula in an area known as The Coves. At the northern tip of the peninsula is another geologic point of interest. A short walk to the lakeshore from the parking lot gives great views of what geologists call The Great Unconformity, a nonconformity that can be traced throughout many parts of the world. Here, older 2.7 billion year old peridotites are in contact with younger 960 million year old Jacobsville Sandstone, a gap in time of about 1.7 billion years.”

Another fine display of one of my favorite places
in the UP. And I agree, Mary...Dr. Nat knows her
stuff.

I agree Alex...... one of my favorite places too.

It's amazing everything is going on with the rocks in that one area. Thanks for showing us around, Dr. Nat!

Great Geology Lesson!! Thanks Dr. Nat!! Anyone
who spends any length of time in Marquette has
visited the Black Rocks and has stories to tell,
especially if you attended NMU. If your
fortunate enough to spend time in Marquette
during the Summer after the school term is over,
this is one of many special locations, along
with the Dead River Falls and the swimming hole
by the falls.
When I think back on all these
memories... It's hard to believe I graduated 40
years ago!!! LOL I'm ready to spend more time
there and 100+ miles further U.P. U.S. 41, Can I
get a •••• YEAH!!