February 16, 2011
When you see layers of rock, i.e., strata, each stratum or layer is a cross section of a former surface of the earth in that location. Portions of the present surface of the earth will someday -- a million years from now or a thousand million, or more -- be a stratum within a stacked layer of rocks. Each layer is called a "horizon." I think that is correct; the presentday horizon will someday be a stratum containing fossils of people, cars, skyscrapers, cities, aircraft carriers and perhaps most of the familiar elements of the present world.
When we think of "fossils," we mostly think of leaves, trilobites, and the bones of fish, dinosaurs and other creatures. But not all fossils are those of plants or animals that fell on the surface of the earth at some past time horizon and then got buried and preserved for millions of years.
About 20 years ago I was driving just west of Hancock, Maryland, on old US route 40 when I noticed an unusual pattern on the wall of a small road cut.
US route 40 still exists but the roadway has been replaced with Interstate 70, four lanes, dual-divided, and the top of the hill where I saw this polygonal pattern in the shale has been trimmed down and the old road cut is long gone.
Usually when you're driving and see something interesting near the road you just keep going. But this time I jammed on the brakes and pulled onto the shoulder and backed up.
I had actually had been heading out that way searching for fossils. There are more of them then ever, because most of the I70 road cuts are deep -- but now itís illegal, and unsafe, I guess, to park and look closely at the exposed rocks.
It was late afternoon. The day was warm. I took these photos while I was wondering what I was looking at. It took about 20 minutes to figure it out -- or rather, in the scientistís way of saying it, to come up with a hypothesis about what could have made those polygonal shapes in that ancient surface of the earth.
The pattern was ten or twelve feet in diameter, but that it was inclined about 60 degrees to the horizontal might have thrown my thinking off. Traffic was zipping by as I leaned against the front fender, watching the traffic but also gazing into the western sky trying to think of how it might have formed. [You can click on the images for larger views.]
My memory -- and this could be a fabrication since this happened -- is that my gaze ran along the paved edge of the road right next to the mostly gravelly shoulder where a puddle was still drying up from recent rain. If the puddle is not a false memory, itís too bad I didnít take a picture of it, too, because, in my unreliable memory, the edges of the puddle were dried and cracked and curled in the way that mud usually dries up and shrinks. At any rate, my hypothesis is that polygonal pattern in the wall of the road cut is a fossil mud puddle, one that had dried long ago in the light of the same sunlight I was standing in, but around the time Europe and North America were just beginning to drift apart.
I have a vague memory, too, of later going to the U.S. Geological Survey in Reston, Virginia, where a specialist in that region told me the puddle dated to ~200-million years ago.
Oh, I also think I remember a tire track going through the new mud puddle, and I definitely remember thinking how neat it would be to find a fossil puddle with a tire track in it.
Fact is, cars have now been around long enough that somewhere in the world there might be a tire track in a mud puddle that will endure 200-million years -- and some fossil hunter of that time might find it and ponder over, and then say, ďOh, yes!Ē