It is a banded, wave-rounded cobble of gneissic rock the size of a small fist. Likely over a billion years old, it is Canadian by birth and Michigander by design. Wet it glistens, dry it glitters: sparkling, alternating, perfectly parallel black and white bands of minerals encircling the surface like the storms of Jupiter.
It has been buried, baked, squeezed, broken, dragged, eroded, scoured and abraded. It has known fire and ice, water and wind, deep time and impatience. It has traveled thousands of miles in the last five million years – a few million spent locked in slowly grinding ice, some number of thousands buried in glacial debris and at least the last thousand or two frolicking in the waves of a sandy Lake Michigan beach.
It was found by a man hiking the beach in the summer of 1998 - the summer of Sosa and McGuire. It was a wet cobble, one amongst many tumbled out of a huge bluff of sand, stone and clay, a recessional moraine ripped open by the constant wind and waves and bitter winter weather of northern Michigan. It was irresistible. The man tucked it into his backpack and took it home to a sunny window in Pennsylvania.
This is its story.
• • •
The notion of Pangaea is one that is familiar to most from some distant science class in high school or college – the existence of an original, primordial super-continent from which all present day continents were formed.
Its existence is well-documented. But in fact, Pangaea is only the most recent super-continent in a dance of continents that extends back billions of years. Entire oceans and continents have arisen, eroded and disappeared, leaving only fragments and remnants that have crushed into each other like debris from a river – leaving land-locked chunks of ancient landscapes isolated in the middle of modern continents. The “Canadian Shield” of northern Canada is one such region. Here are rocks unimaginably old, representing a time when the oceans contained only single-celled life forms and the continents were barren deserts, devoid of life.
Our cobble began as a land mass, either the crustal rocks that cooled from magmas, or the eroded shreds of them that piled up as sediment in its plains and valleys and around its edges. Or both. It’s nearly impossible to know, because as the great train wreck that is mountain building progressed, it crushed and squeezed and heated the older rocks in the middle (and at the bottom) of the pile to the point where they could no longer take it.
Crystalline materials melted, or nearly melted, and the constituent ions of minerals became mobile in the superheated pressure cooker soup. Minerals previously locked into position could now migrate. White quartz aligned with quartz and darker, heavier, iron and magnesium-rich minerals gravitated to their own. Flat bands of minerals spread out perpendicular to the lines of greatest stress and the metamorphic rock gneiss (pronounced “nice”) was formed. The gneiss could extend continuously in every direction for hundreds of miles. Our rock was a piece somewhere in the middle of it. It had likely traveled the globe… but now it was locked in the middle of the Canadian Shield.
Time passed. Slowly the land eroded and the gneiss was exposed at the surface. The expansion of the rock and the freezing and thawing of water in the cracks eventually popped a large boulder free. But our cobble was still in the middle of it.
• • •
Just a few million years ago – a mere heartbeat in the vast expanse of geologic time – climatic conditions caused ice and snow to pile up in northeastern Canada: the Laurentide Ice Sheet. Unable to melt away in the short summers what had accumulated over the long winters, it became deeper and deeper and over thousands of years approached depths measured in miles. It spread outwards under its own weight in every direction like a heaping dollop of thick pancake batter splatted down on a large, flat table. To the south it extended at various times into the present-day states of Nebraska, Illinois and Wisconsin, leaving rubble behind that defined glacial stages: the Nebraskan*, Illinoian and Wisconsinan. It pushed its way to the Atlantic, bulldozing and conveyoring end piles that would eventually become Long Island, Martha’s Vineyard and Nantucket.
As it ground over the landscape, it picked sand and silt and rocks and pebbles and boulders as it went. It grabbed rocks the size of houses and ground them to a fine powder. An unstoppable force, it ground ever outward, ever downward – driven by the force of its own weight. In this manner, the large rock containing our cobble was transported southward, grinding, shoving, pushing, breaking.
In northwestern lower Michigan, a piston of ice over a mile thick drove southward as far as it could reach and then began to melt backwards, leaving a series of recessional moraines behind like corrugations each time it stalled. In a bluff just north of Frankfort, Michigan our rock finally ceased its southward journey in the ice. And there it remained land-locked until the battering waves of Lake Michigan, the ceaseless wind, and time itself, exposed it at the surface where it tumbled to the beach to play in the waves. And then for hundreds, probably thousands of years, it rolled back and forth in the fresh-water surf, letting the abrasive quartz sand grind and polish it smooth and round.
But the story doesn’t end there; the best was yet to come. The gneissic rock rolled back and forth in the surf attaining perfection until the summer of 1998 when the man came along and said: “Hey, look at this!” He rolled it in his hands, put it in his backpack, threw it into the trunk of his car and drove it back to Pennsylvania where it would begin yet another stage in its life: as a teaching specimen.
It would be used to tell stories.
And the story won’t end there either. The cobble has survived a billion years, it will certainly outlive the man who found it. Eventually he will probably give it to someone who admires it. They may set it in their garden and forget about it. It will become buried in the soil, eventually erode and be washed into a river… and off it will go on new adventures.
I envy it.
So there. Now you’ve read about it and you know the story of the small, black and white banded cobble of gneissic rock that rests beside me as I write this. It has been said that history doesn’t begin until someone writes it down. But it’s pretty obvious that the cobble’s history didn’t begin tonight. And now that it’s written, will the story last as long as the rock? Or is the story, and therefore the history, just an ephemeral remnant of the Age of Man. And for that matter, did the rock ever begin and will it ever end?
Or is it simply nature in transit?
* The “Nebraskan” glacial stage is no longer in general use by glacial geologists. I just like the way it fits with the story.