| These rocks are
of Keeweenawn age. It is the same kind of rock that is formed at
ocean rift systems and ocean floors. They form a rift system
a midcontinent gravity high (because of the greater gravitational force
they cause because of their higher density) that goes from Lake
through Minneapolis/St. Paul, central Iowa and into Kansas. It is
reasonable to explain this rift system by having continental plates
to split apart but not completely separate.
In the rocks there are pillow structures and at the top of the rocks are striations, essentially scratch marks left by stones that were carried by glaciers they moved in the NE to SW direction of the shore of Lake Superior.
The rock in the bottom photo has intrusions of calcite (yellow arrows), a mineral that is essentially calcium carbonate. "Intrusions" (really fracture fillings) of calcite seem to be quite common to the basalt in this area.
| This basalt again
intrusions. These intrusions are more visible because the basalt
has weathered along the crack that contained the calcite.
the smaller rocks at the shore are numerous examples of quartz - even
agates, rhyolite, gabbro, limestone and granite. Many of the
rocks have holes, air bubbles that have been filed with other minerals
like calcite, zeolite or quartz (often agate). These are called
It is not uncommon to find pieces of broken glass where the edges have
been abraded smooth giving it the appearance of a rock.
The photo below shows the same rock at the end closer to Duluth. This is a classic example of a Rouche Moutonne' or sheep's back formation. The glacier slowly rises as it advances on the rock until the rock suddenly breaks off and gets incorporated into the glacier. I like to call this a door stop shape.
|This is a sample of basalt with an intrusion of rhyolite. It is at the mouth of the Lester River, on the east side. The rhyolite flows into the basalt (called country rock because it was there first) and then crystallizes.|
|These photos show a gabbro inclusion in basalt. Because gabbro is an intrusive rock (meaning that it is formed under the surface where it can cool slowly to form the larger crystals) it must have broken off the volcano and then carried to the surface in the basalt lava where the basalt cooled quickly. Striations are also visible in these photos but it is difficult to distinguish them from fractures in the rock.|