Motus Mentis

I live in Park Slope, Brooklyn, only about half a block from Prospect Park. Park Slope is aptly named – the land ascends smoothly from the harbor to the park, rising about 150 feet over a couple of miles. There is a low ridge connecting several little hills; the highest of them, and the summit of King’s County, is the vertiginous Battle Hill, about a mile away in Green-Wood Cemetery, piercing the clouds at 220 feet. Brooklyn cannot, however, boast the highest point in New York City – that distinction belongs to the otherwise lowly borough of Staten Island, where the Todt Hill massif soars to a breathtaking 410 feet, an irresistible lure to every Alpinist from the Kill van Kull to Perth Amboy.

Geologically, Brooklyn is quite distinct from nearby Manhattan, where the Precambrian bedrock, billions of years old, lies at or near the surface. The bedrock under South Brooklyn can be as much as a thousand feet down, while the unconsolidated surficial deposits of clay and gravel are of very recent provenance, only a few millennia old. Here’s the story.

The ridge I described above – the crest of Park Slope – is in fact the eroded remnant of a terminal glacial moraine. During the last Ice Age, the Laurentian Ice Sheet, thousands of feet thick, mantled all of North America’s upper latitudes. As it grew it mercilessly gouged and scraped the rock below, shoving debris along before it, and when it receded it left the pile behind. Were I to have stood, ten thousand years ago, at the present site of my charming little bow-front Victorian limestone townhouse, I would have looked out over a glacier thousands of miles wide, with the blue Atlantic at my back.

The land falls away quite steeply as you head southeast through the park, past the people throwing Frisbees at the north end of the Long Meadow, and the Historic Marker of Battle Pass. By the time you get to the Zoo you have descended a hundred feet or so, and from there, heading south, it’s another five miles or so across low and level ground to Coney Island and the the sea. The bedrock is far, far, below the surface here; all this land, the dreary flats of Gravesend and Midwood and New Utrecht and Bensonhurst, was built up on the sea floor as the glacier melted and retreated. As the ice liquefied the water flooded out in great torrents, releasing the cataracts of clay and silt and gravel that became the South Brooklyn outwash plain.

But up on the moraine the view is grand, and we get a nice breeze blowing up Ninth Street from the harbor. You get used to the altitude after a while.