Treasures from the beaches of Sedar Bay (Part 1)

Wayne W Sukow PhD and David L Schuder BA (USA)

This is the first of three articles about the Upper Peninsula of Michigan. Here, there are many miles of Lake Superior shoreline, but the three articles will just cover the Keweenaw Peninsula, where you can find agates, copper, silver, datolite, prehnite, thomsonite and much, much more. However, we will limit our comments to the mineral and fossil treasures of Sedar Bay on the Keweenaw Peninsula (Fig. 1a).

Fig. 1a. The arrow tip points to the location of Sedar Bay. This part of Michigan’s Upper Peninsula is affectionately called “Copper Country” by both local residents and others captured by its beauty and serenity.

Fig. 1b. While this road map is a schematic, it will help. The total distance is about five miles.

Once, it was only forests right up to the sandy and gravelly beaches. Today, considerable stretches have holiday cottages tucked in the woods, with a path to the beach along with a view of Lake Superior through the trees.

Fig. 1c. A beautiful sunset on Sedar Bay.

Today, these residents often cruise the beach looking for pretty stones and the supply seems endless. Everyone collects a few, but some collectors are very discerning and only collect the water tumbled cobbles. Yet, only a few then carry out lapidary work on their finds – slicing, polishing and looking to discover the true nature of the treasures they’ve collected.

Fig. 2a. A close up of a water-sorted row of cobbles that is still wet. It shows the reddish hue of these beach gravels and is where the Lake Superior agates are found.

Sedar Bay is large. It runs about half way up on the north side of the Keweenaw Peninsula and really starts about eight to ten miles north of McLain State Park. It then runs northeast along the Keweenaw Peninsula to about the mouth of the Black River, which is just south of the mouth of the Gratiot River. The bay was named after an early settler, who set up a farm there. One of the authors (David) enjoyed summers there with his mother and father.

Fig. 2b. A view to the southwest of Sedar Bay, which is back toward McLain Park. These rows of different sized cobbles seem to be endless to a newcomer to the beach.

His father, Dr Schuder, was a professor of entomology at Purdue University and a master mineral collector, and David continues that tradition. In contrast, Wayne has been visiting the Keweenaw Peninsula for the past 50 years, but had never visited Sedar Bay to collect agate and other treasures from its beaches since it required crossing private property. That has now changed, as some homeowners will give permission.

Fig. 2c. A view to the northeast along the beach, with a deposit of fresh rows of cobbles, stretching from the foreground to a bend in the beach nearly a half mile away.

The map in Figs. 1a and 1b orient you to the locale and Fig. 3 is a view along the beach during a ‘big blow’. These storms stir up the cobbles at the bottom of Lake Superior and continually provide new material to collect (Figs. 2b and 2c). As you might guess, the local residents have the advantage, but the really good collecting starts from about the end of the Tamarack Water Works Road and runs northeast to the mouth of the Black River.

Fig. 3. A ‘big blow’, showing a storm that sends waves crashing onto the beach and brings in new gravels from further out in the bay.

The geological background to Sedar Bay agates, minerals and fossils

If you want to understand Lake Superior agates, minerals and fossils, you need to understand the geology and the starting point is the ‘Mid Continent Rift’ (MCR). The origins of the MCR date back more than a billion years. Some scientists believe that it was initiated by the arrival of an anomalously hot mantle plume at the base of the lithosphere, about 1.10 billion years ago. (A mantle plume is a bulging lava chamber that rises above the rest of the earth’s mantle, thereby heating the rocks above it to much higher temperatures than usual.) The rift begins in north-eastern Kansas, trends north-eastward to Duluth and then begins a slow bend between Isle Royale and the Keweenaw Peninsula. Finally, it appears to go below Lake Superior and continues trending southward through the Lower Peninsula of Michigan. The length of the rift is in excess of 1,562 miles, which is continental in size.

Today, it is recognised that three categories of geological events shaped the rift’s character, and these explain the rifting and ancient lava flows of the Keweenaw Peninsula that occurred during the Mesoproterozoic Era:

A thick layering of flood basalts at the earth’s surface.
Local concentrations of intrusive rocks.
Upper sequences of flood basalt flows and sedimentary rocks.

In the Geological Survey Michigan, 1881-1893, there is a plate showing the geological bedding of the north Tamarack Shaft No3. It identifies no less than 77 layers of trap rock (flood basalt flows) alternating with amygdaloidal basalt flows and conglomerate layers. Even then, the anticipated flat sandstone bedrock had not been reached. Such multiple beds of lava are clear evidence of the multiple flood lava flows.

Eventually, the rift became inactive and this allowed for the cooling of the multiple layers of dense lava basalts and andesite, which caused downward warping of the rift valley. That depression in the earth’s surface began the creation of the Lake Superior basin. As rain fell onto the rift valley, the higher and steeper walls associated with faulting at the edges (for example, the Keweenaw and Isle Royale faults) produced sediments that gathered in the valleys of the rift.

Today, seismic profiles across Lake Superior and gravimetric data indicate that the deepest part of the MCR lies beneath western Lake Superior and contains as much as 30km of fill, with volcanic rocks comprising about two-thirds of the total. We suggest that such sand, gravel and boulder filled holes may be an important source of the chalcedony cobbles found on Sedar Bay beaches.

During their advances, the continental glaciers scoured the basin and mixed the accumulated sand, gravel and boulder sediments. Also, during the ice age, as continental glaciers more than a mile thick advanced and retreated across the Lake Superior basin, their weight caused even more downward warping of the rift valley on its southern boundary. At the same time, the glaciers blocked lower elevation outlets on the eastern edge of the lake forcing the level of the lake to increase dramatically. For a while, the Lake Superior Basin drained southward from its western end through the proto-river valleys (the St Croix and Mississippi) in the narrow corridor between Wisconsin and Minnesota. We believe that today, the Lake Superior agates are found in the glacial tills in this corridor as a result of this sequence of events.

Then, during their retreat, these continental glaciers unloaded the sand, gravel and boulders they were carrying in moraines and valleys. These moraines and the outwash fans from the retreating continental glaciers, such as the very last one – the Wisconsin Glacier – also scoured deep trenches into the emerging lake’s bottom. These are unusually deep and long bedrock gouges in the eastern basin of Lake Superior. They are really small canyons reaching in the order of one and a half miles in width and tens to hundreds of yards deep – a testament to the strength and fury of the glacial meltwaters.

The mini-canyons have been interpreted in a variety of ways. They may be a south-trending stream system, formed during glacial or interglacial periods; or they may be pre-existing fractures that were eroded by glacial ice and meltwater. Recently, the ‘Blue Heron Trough’ was the subject of an extensive geophysical investigation. It showed that these small canyons may be partly filled with lake sediment from an earlier epoch and are also filled by debris slumping from the canyon walls. In addition, wedge-shaped accumulations of sediment were found that cross the canyons in several places. These could be the equivalent of either glacial outwash fans/plains composed of cobbles and gravel; or gravel deposited by meltwater under the glaciers; or both. Regardless of the explanation, they would ultimately be a second source of Sedar Bay mineral treasures.

From the data, some scientists believe the mini-canyons were formed by south-flowing, sub-glacial streams, flowing directly toward the shores of the Keweenaw Peninsula and Sedar Bay. These mini-canyons have cut more than 100 yards into the flat bedrock of the lake bottom. That suggests that their walls also expose layers of the MCR lava flows, including the amygdaloidal flows and conglomerate layers. Some of these flows have rich agate beds, such as those exposed at Michopten Island.

Other lava flows, containing beautiful agates, have recently been discovered by Bob Barron of Chassell, MI, who is a well-known field collector of Copper Country minerals. Those beds are under the water in Lake Superior and off the shores of Keweenaw County. It has also been estimated that the agate out of the MCR (Lake Superior and Sedar Bay) may be some of the oldest agate in the world.

The second article covers collecting from the region, while the third deals with fossil collecting.

Acknowledgements

All but a few of the Sedar Bay mineral treasures shown in this article were collected by Dr Donald Schuder (deceased), who taught at Purdue University, his wife Mary and his son, David Schuder, who is co-author of this article. Also, Sam Norwood’s Sedar Bay collection is well represented.

We are indebted to Dr George Robinson, curator of the Seaman Mineral Museum at Michigan Technological University, Dr Gene LaBerge, geologist at the University of Wisconsin-Oshkosh, and Tom Rosmeyer, the former geologist at the Camp Bird Gold Mine in Colorado. Thanks also go to John Marshall, who started all this a few years back and his editing skills. All photo images are by David Schuder.