Bill Rose

The Jacobsville is the youngest of the Keweenawan Supergroup, rock units that filled the mid continent rift of Rodinia beginning at least 1.1 billion years ago and continuing for ~200 million years. The Rifting of Rodinia (a giant supercontinent which was made up of pieces of all presently known continents) resulted in huge lava flows, bigger than any that have come since. These are the volcanic formations shown to the right.   After volcanism slowed, sedimentary rocks flowed into the rift from the steep sides and erosion finally diminished the mountainous landscapes to hills and with that change the sediments evolved from conglomerates to sandstones and shales. Outside of Michigan the formation has different names, such as Bayfield Group (Wisconsin).

The red sandstone now called “Jacobsville” is a spectacular part of Keweenaw geology, making up much of the SE side of the Peninsula, south of the great Keweenaw Fault.  The rock was quarried and used in many of the most impressive buildings.

Sandstone is made of sand particles.  Sand particles range in diameter from 0.0625mm (or 116 mm, or 62.5 micrometers) to 2 millimeters.

Pt Abbaye
Great Lakes Shoreviewer