They tilt, in one direction or another. On College Hill in Easton, they dip to the southeast. This is one clue to decoding the riddle of the land we occupy for the tiny speck of time that is the present. We are within the puzzle, and typically we walk around oblivious of the signs of the millions of life forms and epic past traumas that are in plain sight. But last Saturday was different. Last Saturday, a small group of humans, including me, moved, or should I say younged, down the hill from the Lafayette College campus with David, one of its geology professors.
The layer cake (David’s term) of sedimentation that was College Hill had been shoved rudely on its side as tectonic plates slid and crashed couple of hundred million years ago, as Africa split from North America. This explains why the rocks downslope were younger than those higher on College Hill—and why we were younging rather than, I’m guessing, olding. The ridge we examined was put in place 520 million years ago, give or take a few hundreds of thousands, layer by layer. That’s what the fossils and chemicals tell us. Some time later (a million years, a couple of hundred million years … to our shopping-, holiday-, work-driven lives, what does it matter?) the rocks relaxed. Cracks developed perpendicular to the attitude and became filled with water and all that it carried. Like marbled meat, many of the dark rocks are veined with white. Here are some of David’s secrets about reading the rocks:
- The fine grain means that this was, at the time it was forming, a “low-energy environment,” meaning a still lake, or a deep ocean.
- We see a few stromatolites, formed by microbes deep, but not too deep, on the ocean floor. They were deep enough that there were no rough currents causing large grains to settle into the sediment, but shallow enough that the microbes could have the access to the sun they need to survive and grow. As the microbes increased in size, grains of mud settled in around them, and bumps developed in the sediment. In cross-section, they look like knots in a piece of pine. And …
- Nothing ate the microbes. This means there were not a lot of animals around at the time.
- We did not actually do the “acid test,” but David assured us that the rocks we were looking at do indeed fizz when acid is dropped on them. This means the rock is composed of carbonates, not to be confused with plant carbon. Carbonates tend to accumulate where it is warm and shallow, said David, which aligns with the fact that Pennsylvania, during the Cambrian era, was much closer to the equator that it is today.
- Within the strata are cherts, blobs of, in this case, black. Roughly at the time the rocks were formed by sediment, silica precipitated out and the hard black mineral materialized within the sediment … as cherts.
Putting the clues together, we are looking at a tropical time. Animals were rare, the sea was shallow, and the earth arranged itself in layers over time, but not at a steady pace. Long periods of stability were punctuated by bursts of energy, and sediment was scoured from the hills in fits and starts. A box of tissue sits unused on the kitchen counter until a virus causes the immune system to go temporarily haywire. Trash is put out and collected day after day, and all is well until a violent wind blows and plastic bottles rattle down the road, bags fly into the trees. These are examples of punctuated equilibrium, on our human scale. So, younging along (the very word lifts the spirits), we moved 60 meters, and tens of thousands of years, down the road, and there we see a dip slope. The sea floor has tilted in this Bahamian world where mountains rise and fall and animals are scarce. The foundation of the house at the base of the slope tells yet another story. Rip-up clasts, pieces of rock tossed by currents or violent storms, landed in the sediment 520 million years ago (or so) and so the wildness became locked in the rock.
The very concept of geologic time with its 500 million years of deposition, with its folding and uplifting and upsetting of the layer cake 200 million years later, with its durable fossils and hints of storms, makes our lives feel very small. We live our lives, our loved ones die, we are thrown into a mighty turmoil. And yet, in the language of the rocks, we are not even a whisper.
Rocks have attitude. They have mass and history. Seemingly timeless they are subject to forces that we may fathom, but not feel. 520 million years from now, we will have become fossils trapped in a layer, and the rocks will fizz with our stories.