Lake Geology

Fire and Ice

Fire and ice, in the form of volcanoes and glaciers, created Lake Superior. That happened long before Jake came here to fish. One billion years ago, molten basalt erupted from the mid-continent rift, which extended from near Thunder Bay, Ontario, to Kansas. Lava flowed from the rift for 100 million years, resulting in a layer of basalt as much as five miles thick. Billions of tons of molten rock, moving from underground, caused the surface to gradually sink and form the Superior Basin. The basin remained dry and level until about one million years ago, when glacial ice began to sculpt it into roughly its present shape. Only ten thousand years ago did the last ice mass begin to retreat. Impounded water formed what is now called Glacial Lake Duluth. The surface of this lake was five to six hundred feet above the present lake level. Duluth’s Skyline Drive, with its wave-cut cliffs and terraces, remains one of the highest former shore lines of Glacial Lake Duluth.

The north shore of the lake has the most rugged terrain. High bluffs and irregular shorelines composed of volcanic rock resist erosion by waves, but freezing and thawing break them into boulders, and these are ground into sand and clay.

Our shoreline is made of softer lava flows over a hard, dark basalt. You can best see the flows at the west end of the resort, just up from the old commercial fishing dock. Veins of minerals can be found in the cracks which formed when the basalt cooled after it had bulged up from deep within the earth a billion years ago. Thomsonite crystals formed in these cracks along with other zeolites. Agates as well as thomsonites formed in the holes left behind by gas bubbles in the hot rock. And as waves tumble and wear the rocks away, these attractive gems become part of the gravel and sand.

Storms and waves

With such a large stretch of water, the waves can grow to large sizes when the wind blows hard and steady for several days. Late fall and early winter have the strongest winds out of the east and northeast, and our biggest storms are often accompanied by lake-effect snow. Waves can be 12 to 15 feet. Old Bob claimed that he once watched 20-foot waves come ashore and wash solid water under the cabins at the east end of the resort.

High water levels in recent years have made storm-wave erosion of clay banks above the bedrock a serious problem for the North Shore, as well as an erosion and flooding problem for all the Great Lakes. Over the years, the lake level fluctuates with changes in climate and varies about two feet above and below the average of 600 feet above sea level. A few drier years now have brought the lake down from 602 feet to just over 600 feet.

One reason waves can grow big on Lake Superior is because the water is deep, often right up to the shore. Out from Bob’s Cabins there is an old boulder beach (boulders strewn on solid bedrock) flooded to a depth of 15 to 25 feet. It extends out to 200 to 300 feet, and then the bottom drops away regularly for another half mile where there is a 600-foot deep trench that extends most of the way along the North Shore. The fishing boats often troll along the edges of this trench in about 200 feet of water.

When winds shift, waves may come to shore from two or three different directions because the waves may be traveling from many miles away. Watch for the wave patterns and the taller “rogue” waves created when two waves cross each other and combine their sizes.

Some people are curious whether we have tides on Lake Superior. Yes, we do, but they are so small that they are not observable. About an inch of rise or fall because of the moon and sun’s gravity is all that can be expected. However, we have another water lever fluctuation called a seiche (pronounced saysh), which can amount to about one foot on this lake. It is caused by atmospheric pressure (high at one end of the lake and low at the other), and by wind blowing in the right direction, which piles water up at one end of the lake, lowering it at the other.