North Shore Rocks 

Here at the shore of Lake Superior adjacent to Leif Erickson Park we find evidence of a totally different kind of rock in this area. It is a sedimentary rock formed from sand and basalt. The color varies from purple to red to green. The entire natural layer is about 100 feet thick and indicates a very long pause in volcanic activity in this area. To form the sandstone of this thickness erosion would need to take place for a long time between the volcanic erruptions. This sand from the long period of erosion then could be heated and compressed into the sandstone we now observe. The particles of the sand are mostly fragments of basalt (like the black sands you might find on the black sand beaches of Hawaii). This means that the source of the sand must be rather local, coming from the same basalts as the basaltic rocks caused by the midcontinent rift. The green color comes from oxidation of epidote. Epidote is a moderately hight temperature (200-350°C) hydrothermal (hot water) mineral that indicates heating due to deep burial within a volcanic pile.

The zig-zag or sidways "v" shape layers of the sandstone that get cut off at intervals are called cross bedding. This kind of layering happens when the sand is moving. The motion of the sand is caused either by wind or by moving water. Since this was not a desert area as far as can be determined the crossbeding indicates that moving water must have carried the sediments here before they were compressed into rock. The presence of epidote is futher evidence of the presence of hot water.

People often wonder "how do they know". This is an excellent and important scientifitc question. In any scientific explaination there must be evidence. In the photo at the right we have some remains of this scientific evidence. Some geologist has drilled holes in the rock to collect samples. These samples can be tested for mineral composition, for mineral size for thickness of rock layers and even tested for age by radioactive dating. The photos below show a pahoehoe (ropy) flow of rhyolite. Rhyolite is a rock of the same mineral composition as granite but the mineral crystals are much smaller, not visible to the naked eye. The smaller minerals are formed when the lava cools quickly where larger mineral crystals have time to form when the mineral is cooled much more slowly. These rocks are visible at the Lake Superior shoreline northeast of Brighton Beach on Scenic Highway 61. You'll need to take the 3rd pulloff northeast of the park entrance and walk back about 100 yards through brush to make it down to the beach. If you go try to find cylinders of amygdules (mineral filled gas holes called vesicles) in the interiors of some of the basaltic flows.

The photos at the right and below are at the Silver Creek Tunnel. There is a parking area at the northeast end of the tunnel and a paved path that one can take at the lake side of the cliff that used to be the roadway. The rocks here are diabase (mineral grain size between that of basalt and gabbro but the same compostion of both basalt and gabbro) and are at least 200 feet thick. The rock was formed when basaltic minerals intruded into the rock that was already present. This diabase rock is much harder than the original basalt causing it to errode much less. The result is a high cliff above Lake Superior that was able to withstand the effects of glaciers and other erosion. The tunnel has exposed the contact between the diabase and adjacent bolcanic rocks. The photos on the right show a north striking, 55° east dipping brittle fault that cuts the base of the diabase. Below the fault is and andesitic flow showing a very irregular and chaotic flow contact. It contains quartz-lined stretched vesicles and amygdules of gray agates that have been recrystallized by contact metamorphism from the adjacent diabase. The margins of the diabase are marked by a mix of comingled fine-grained, strongly magnetic dark gray diabase and pink granite. The fault line is about three meters thick and filled with a mixture of pink zeolites and calcite-filling voids around the altered diabase. Out on the old roadbed teh diabase has prominent columnar joints taht plunge approximately 60° east as well as excellent views of the Lake Superior coast
The photo at the right is evidence of multiple lava flows. I'm sure (at least I hope) you wonder why some points and bays in a large lake can be evidence of these multiple flows. When the lava flows out from a volcano the gases trapped in the lava float to the top. This means that at the bottom of the flows there are very few vesicles (gas holes) causing the rock to be much harder. At the top where there are many vesicles the rock is easier to errode. Because the lava flows are tilted slightly to the south each point in the lake is caused by the harder to errode bottom of a lava flow and each bay is caused by an easier to errode visicular lava flow further up in the layers. Each point and bay then are evidenced that geologists can use to help locate the next lava flow, going up in the layers the further northeast along the lake one travels.
In the photos above at Gooseberry Falls State Park we have further evidence of multiple lava flows and the resulting erosion that leaves us with the falls. The rock that forms Gooseberry Falls is basalt, the dominant rock type along the North Shore of Lake Superior. Water falls are formed when hard, difficult to erode rock gets broken off exposing easier to errode rock underneath. At Gooseberry Falls the kind of rock is the same but it is the presence of vessicles that cause the rock to be easier to erode. The different layers (steps) of Gooseberry Falls are caused by the multiple layers of lava. If you go to Gooseberry Falls look for some of these features. In the photo at the upper right we are standing at the base of one layer of lava while looking at a tunnel formed in the easier to erode rock. Many of the vesicles are filled with minerals. The filled vesicles are called amygdules and the rock being basalt is called an amygdaloidal basalt. In the photo at the top left we can see a polygonal columnar joint. These vertical, often five or six sided columns are typically formed during the cooling of massive (thick) lava flows.