Vacationing for Rocks
Brenda and I are off to visit the U.P., something we’ve never done except in transit to Canada. Peace and quiet await. And rocks, which has me thinking about the geology of Michigan.
Between 400 to 350 million years ago the North American continent looked very different. Much of it often was underwater, as so-called shallow seas advanced and retreated. The familiar mitten shape of the Michigan lower peninsula was once a bowl-shaped basin containing an equatorial, paleozoic ocean full of trilobites and corals. Over hundreds of millions of years that bowl has gradually filled, first with the coal-like sediment of the earliest trees, and more recently with the gravelly backfill of the last ice age, so that the oldest bedrock of the lower peninsula is, in places, buried three miles down. The bedrock rises travelling north, finally reaching the surface in the upper peninsula where it becomes part of the vast Canadian Shield. Some of the oldest exposed rock on Earth is found there. The magnificent Niagara escarpment is a shelf of limestone at the southern edge of the shield that has resisted millions of years of erosion, forming an arc that you can trace on a map, originating near the Niagara falls (naturally), extending northwest and then westward as the Bruce peninsula of Canada and Manitoulin Island, then continuing around the southern border of the Michigan UP before forming the Garden and Door peninsulas of Wisconsin, which themselves are the gateway to Green Bay.
Much more recently, if 20,000 years can be considered recent, glacial ice was a mile thick over present-day Michigan, extending as far south as the middle of Ohio. By 15,000 years ago, as the ice began to retreat, the familiar shape of the lower peninsula began to emerge. At some point the glaciers retreated behind the moraines that form the boundaries of the Mississippi river drainage basin, trapping the meltwater where it began to fill the Great Lakes basins.
The Farmhouse sits on land that was dune sand thousands of years ago, but has gradually settled into a more solid form. To the extent the Farmhouse was ever part of a farm, I’m guessing it was a fruit farm. Apple and cherry orchards are a dominant crop in this area, and asparagus, weirdly enough. I guess it has to come from somewhere. The mix of sand and loam makes great dirt for growing things. If it just wasn’t so cold for so long in the spring!
Just to our west the more recently formed and still shifting barrier dunes define the current Lake Michigan shoreline. The churn of ice and waves make the Great Lakes shorelines a treasure chest for rockhounds. On the Lake Michigan shores near the Farmhouse we can find well-rounded cobbles of basalt and granite, and maybe a lucky bit of quartz or bloodstone. For me, one of the most unusual Michigan “rocks” is not stone at all but comes from the remnants of 19th century iron smelting operations that dotted the lakeshore. These ironworks dumped molten slag directly into the water, where it cooled into chunks of colored glass. If you search long enough, you can finds bits of blue or green slag washed ashore near Leland or Frankfort.
The shorelines of Lake Superior, being much closer to ancient bedrock, are hunting grounds for iron-red agates, and more recently are being scoured for so called yooperlites: chunks of granite-like rock that contain inclusions of sodalite, which glows yellow under ultraviolet light. And, of course, there are the famous Petoskey stones, the fossilized remains of coral that used to populate those ancient shallow seas. If you visit Michigan and choose not to walk the shoreline looking for your own Petoskey stones, rest assured that the smallest shack billing itself as a rock shop will have several polished specimens ready for you to buy.
We have our UV flashlights, our hotel reservations, and nothing but a week of time. I’m hoping to find some rocks. If not, there’s always the rock shop.