By Kevin Selkregg
The Great Lakes
The shores of the Great Lakes are deeply revealing to both the natural and man-made transformations these freshwater inland seas have undergone recently and deep in the past. Because of the diverse array of treasures that wash up along these shores, beachcombing is a favorite occupation of many folks. There is fascinating beach glass, metamorphic and igneous rocks transported to the lake shores from glacial activities, and fossils from prehistoric organisms.
The Great Lakes formed only a few thousand years ago, yet they are a result of much older geologic processes. Think of the geology of this region as a “cake” with three basic layers of fundamentally different rock types and ages.
The lowest layer, which geologists informally call the “basement,” is metamorphic rock billions of years old. This rock is exposed in the Adirondacks, the Upper Peninsula of Michigan, and across much of Canada. Most of the hard, multicolored, and banded cobbles and boulders that the glaciers imported southward to our shores come from this section. Ranging from pebble-sized rocks to large boulders, and rounded by wave erosion, these colorful rocks with a variety of banding textures and mineral accumulations can be found on Great Lakes beaches.
The basement is separated from the next higher layer by a sharp, irregular boundary of rock, which is no longer present due to erosion, and represents a billion years or more of intervening time. Called a nonconformity by geologists, this time-gap boundary separates the basement from the overlying layers of marine (salt water) and nonmarine (fresh water) deposits. Ranging from 300 to 500 million years in age, these layers record the evolution of plants and animals, as well as the migration of our continental plate across the globe. The top of this layer is the bedrock surface across most of New York, Ohio, and lower Michigan.
The third layer overlying the bedrock consists of a superficial “drift” cover, laid down by glaciers moving across Canada, the Great Lakes, and the Upper Midwest. Ranging from 10,000 to 30,000 years in age, this section of loose rock reflects the last major glacial phase of the “ice age.”
Many of the fascinating beach finds along the Great Lakes aren’t rocks—there are actually fossils. Fossils are nature’s way of revealing evidence of prehistoric organisms, and the evidence in this region comes from a time long ago before the Great Lakes formed.
Generally only organisms with hard body-parts became fossilized, leaving depressions or molds in rock that were filled with minerals over time, or that were preserved in their original form—a natural embalming of sorts. For fossil formation, an organism must be rapidly buried before it decomposes or is eaten. Some fossils are remnants of relatively recently-perished organisms that may not have been altered at all, such as teeth, bones, or shells. All of these fossil types help us interpret and understand conditions in prehistoric environments.
The record of the geologic past as presented by fossils is biased because only certain environments provide these conditions of preservation. The population of organisms preserved as fossils represent a tiny fraction (5-10%) of all those that have lived in the past, with the vast majority of fossils originating from the shallow-water marine environment.
A stone or a fossil?
For the beachcombers trekking along the shores of the Great Lakes today, what kinds of fossil evidence can they expect to find? Along the shores of Michigan’s lower peninsula, one can be rewarded in finding Michigan’s official state stone, the Petoskey stone. Technically a fossilized colony coral, this fossil consists of six-sided corallites—skeletons of once living coral polyps. The Petoskey stone and its cousin, the Charlevoix stone, are found along Lake Michigan’s western shore from the northern tip of the lower peninsula to as far south as Manistee. These are originally formed in approximately 400-million-year-old limestone rock in Northern Michigan and later transported south by glaciers. These finds indicate a warm, shallow salt-water coral-reef environment that you would typically find today near the equator. But the Great Lakes basin is nowhere near such an environment, so how could such a fossil be possible?
Will Durant once said, “Civilization exists by geological consent, subject to change without notice.” This statement is obvious for those of us who have experienced rapid geologic events like a severe earthquake or lived at the base of an active volcano. However, the continents don’t experience rapid change, having moved very slowly to their current locations, at a rate of a few inches per year. The earth’s crust is not one single mass but a menagerie of fractured plates shifting and moving continually due to subsurface forces. This is known as “continental drift” or the more modern term “plate tectonics” used today.
Approximately 360–420 million years ago, all land masses were clustered together in their early form on one side of the globe with the rest of the globe one large ocean. The early North American continent at that time was actually located just south of the equatorial region, with Duluth, Minnesota, near the equator (red circle below). This warm climate, along with the existence of a shallow inland ocean (in the geographic region now occupied by the Great Lakes) led to a marine life much like that seen in the coral reefs of the Caribbean. This era, called the Paleozoic, was a time when corals, mollusks, trilobites, and brachiopods inhabited a warm and fertile ocean. According to Jerry Dennis in The Windward Shore: A Winter on the Great Lakes, this sea nurtured a “profusion of plants and animals so rich that when they died and sank to the bottom they formed limestone beds hundreds of feet thick.” Over the next 400 million years, this landmass cluster began to break apart, increasing in size and spreading out in the planet’s current arrangement of continents. This movement included the North American continent shifting to the Northern Hemisphere with a slight counterclockwise rotation.
Sedimentary rock types such as mudstones, sandstones, shales, and carbonate rock, along with their embedded fossils lay undisturbed in the marine and non-marine deposits layer until glaciation began about 2.6 million years ago, shaping the surface topography of the Great Lakes Basin. When the glaciers dug into the deep layers of sediment where the fossils rested, the remains were released and transported to the lake basins. Acting like giant conveyor belts, these glaciers transported rocks and fossils south to much of the lower peninsula of Michigan and on to Ontario, Ohio, Pennsylvania, and New York beaches—all teeming with fossils along their lake shores. Today, the constant wave action of the lakes either deposits them on the beaches or erodes them from the shoreline; all for beachcombers to find.
Great Lake Fossils
All the possible fossils one might find would be too numerous to list here. So instead, we’ll cover some of the more common fossil finds by Great Lake beachcombers.
These ancient creatures thrived during the Paleozoic Era. Sometimes called lamp shells, they are some of the most easily recognized fossils, usually embedded within shale slab layers. They are often released from shoreline shale banks rather than washing onto the beach from the lake. One recognizable type is the distinctive approximately 400-million-year-old fossil with a “winged creature” appearance. Beachcombers can find these by breaking apart flakey shale slabs to find the brachiopods between the layers or washed up and smooth on the beach.
Bryozoans (“moss animals”) act as filter feeders that sieve food particles out of the water. They can live as fans or branching twigs and are important components in forming reefs. Fossils of these organisms are found embedded in shales or dolomite rock rather than as single fossil segments.
Chain Coral (Halysites)
These fossils reveal interlocking strings of delicate corals with small tubes where jelly-like coral polyps resided, usually building limestone reef structures on the seabed. They are sometimes found along the shore embedded in dolomite or limestone rock, resembling a seam on a baseball.
Commonly known as sea lilies, even though they are animals, crinoids superficially resemble plants that attach themselves to substrates on the ocean floor. They are famous for their feathery, tentacle-like appendages that open like a flower to filter feed on small particles of food such as plankton. The stems, composed of discs, usually are broken into cheerio-like segments that are the most common find.
Favosite (Honeycomb Coral)
This coral forms calcitic tubes packed together in large colonies with a honeycomb fashion. As a living organism, they were often draped over a rock in a lace pattern. These can be found as irregular chunks along the beaches.
These corals resemble cow horns with a wide head and very narrow bottom and are characterized by external growth bands much like tree rings. These are found in some localities along the Lake Erie shores of Ohio, either alone or embedded within bedrock yet to be released by erosion at the shoreline.
Petoskey / Charlevoix stone
Though the name doesn’t indicate it, the Petoskey stone is a fossilized colonial coral that lived in the warm shallow seas approximately 350 million years ago. The official state stone of Michigan, it is remarkably similar to its smaller cousin, the Charlevoix stone, which may actually be the coral Favosites in pebble form. These stones, rounded and smoothed by wave erosion, are frequently found in the Northern Michigan areas of Harbor Springs, Petoskey, Charlevoix, Northport, Glen Arbor, and Empire.
Looking much like the present-day horseshoe crabs, these now-extinct animals had a body consisting of three parts; a head, a thorax with multiple segments, and a tail. They likely spent their life on or just above the sea floor, scavenging for organic detritus. They can be found from Alpena, Michigan, to Buffalo, New York, along the Michigan side of Lake Huron and all along through Lakes Erie and Ontario, mostly found embedded in shale slabs between layers of the rock on the shoreline.
Read about rocks and minerals found on Great Lakes beaches.
This article appeared in the Beachcombing Magazine July/August 2021 issue.