The stony ground and prominent rock formations of Pound Ridge in Westchester County, New York elicit curiosity in many people, some of whom attended our recent guided hike focused on geology. I will summarize our tour of the geologic history that can be seen at the Bye Preserve, owned by the Pound Ridge Land Conservancy (link to map and directions). Trails at the Preserve are open everyday from dawn until dusk.
Imagine that your arm represents geologic time, with your armpit being the Big Bang and the tip of your middle finger representing today. The dawn of life occurs just above your wrist. Between your wrist and the middle of your palm, all of the bedrock that shapes this land was formed by a process known as the tectonic cycle: erosion of mountains, deposition of materials in layers, subduction (movement of the earth’s crust), and volcanism. There was something on the earth’s crust prior to the formation of our bedrock, of course, but we cannot see evidence of that now. Geologists have discerned that an ancient Acadian Mountain Range once rose east of us, to the height of the Himalayas, and then eroded into particles that were later metamorphosed into our bedrock. Add folding from the movement of tectonic plates, deformation by glaciers, and erosion, and you have a punctuated record of history etched on the landscape.
In its western half, the Bye Preserve is underlain by Fordham gneiss, which formed in the Precambrian era, 1.1 billion years ago. This layer measures up to 500 feet thick and is extremely resistant to weathering. There are many kinds of gneiss, and it is variable in color from brown to buff and can be pink or green. Fordham gneiss contains quartz, biotite mica, silicates, garnet, and other minerals. Bedrock along the eastern side of Bye belongs to the Hartland Formation, which is half as old and is a mixture of gneisses, schists, and amphibolites. It formed when shales and sandstones from a deep ocean metamorphosed and retains its layers of alternating color. Many of these rocks were inverted when the Hartland Boundary Fault displaced older layers and brought in new rocks.
Subsequent to the last great tectonic shifts in the Paleozoic era, the bedrock at Bye has endured at least four episodes of folding and associated metamorphism, which is visible in many rocks where layers are curved rather than straight. The first stage of folding produced large ENE-WSW trending ridges and valleys, while the second stage trended NNE. Later stages produced tight folds that reoriented earlier folds. Where rock is resistant to bending, brittle faulting may occur, which produced the Mill River Gorge just northeast of Bye Preserve and the Mianus River Gorge as well. In the midst of these changes, magma intruded the bedrock at Bye and produced a very pretty white-to-pink granite that is rich in quartz and mica.
More recently in geologic time, between your furthest knuckle and fingertip on our imaginary time scale, the glaciers of the Ice Age left their mark. An ice sheet more than one mile thick ground back and forth across this landscape for nearly 100,000 years in freeze/thaw cycles. Each advance brought in new rock from afar. Each period of retreat caused mass deposition, flooding, and a rise in sea levels. Bye Preserve is now only ten miles from the Long Island Sound and was at times submerged by ocean, although fresh water has played a greater role in carving this landscape. We can see evidence that great torrents once ran in the boulder fields that line two now-intermittent streams, scouring away soil such that even today, only ferns and moss grow there.
The last glacial ice disappeared from Pound Ridge about 12,0000 years ago, leaving behind piles of sand, rock, and even huge boulders on the surface of our scoured bedrock. These are the clues from which we can piece together millions of years of history on the land. To end our story, I invite you to sit with a favorite rock and to ponder what it has endured to bring you both to this moment in time.