The late Pleistocene

Anita lost her balance and almost went below. “What a klutz,” she remarked. I thought I might be learning a geologic term. “These are periglacial boulders,” she went on. “They’re not erratics. They haven’t really moved. In the climates we have now, big boulders that are not erratics just don’t appear in the woods. Only a remarkable set of conditions would produce this scene. You had to start with the right bedrock. You had to have the right angle of dip, the right erosional shape for flushing, the right distance from the glacier. The co-working space almere terminus of the glacier was about half a mile away. The climate was arctic. Imagine the frost heaves after water in summer got into the bedrock and that kind of winter came to explode it. Gravels, sands, and clay were completely flushed away by meltwater, leaving these boulders.”
Twenty-five thousand years ago-in the late Pleistocene, or, relatively speaking, the geologic present-arctic frost had broken out the boulders and had begun the weathering that rounded them. The Acadian mountains, wearing away three hundred and fifty million years ago, had provided the material of which the boulders were made. Back on the interstate and continuing across the Pocono Plateau, we ran through more flat-lying red strata of the same approximate age, and Anita said, “Remember the Bloomsburg? This rock is fifty million years later, and it looks like the Bloomsburg, and it was formed on another low, alluviated coastal plain, when the Acadian mountains were dying down. As I’ve been telling you, geology is predictable once you learn a few facts. Geology repeats itself all through the rock column.” On the geologic time scale, anyone could assign these events to their respective places and sense the rhythms of the cycles-rock cycles, glacial cycles, orogenic co-working space amsterdam cycles: overlapping figures in the rock. Taken all together, though, they seemed to ask somewhat more than they answered, to reveal less than nature kept concealed. The evidence showed that the Acadian mountains had come down, as had the Taconics before them, and each had spread westward new worlds of debris. One could also discern that the Acadian Orogeny had folded and faulted the sedimentary rock that had formed from the grit of the earlier mountains, and metamorphosed the rock as well-changed the shales into slates, the sandstones into quartzites, the limestones and dolomites into marble.