Posts Tagged 'Geology'

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There’s your beautiful world, NW edition. Here’s Masha Gessen, an old hand at autocracy, on surviving Trumpism, very necessary reading now.

Traces of the Ice Age

striationsDon’t you just love these? These grooves are found along the path in the forest of the NYBG, and time and generations of feet have worn them down slightly. They’re glacial striations, gouged out by the rubble on the bottom the ice as it scraped across the hard surface rock.

These can be found in Central Park, too. But not here in the home borough, which is all glacial deposit–made up, come to think of it, with some of that Bronx rock.

Pothole

IMG_6951Seemingly drilled into the schist of Inwood Hill by some kind of large-bore drill, this is actually a glacial pothole, scoured out by the mighty power of swirling water and abrasive stones during the heady days of the Wisconsin glaciation. The diameter is a little over a foot and a half.

The heights of Inwood, very like a whale from the vantage point of the Hudson, are the northern prong of the Fort Washington Ridge. Geologically, this rock is known as the Manhattan Formation, made of mica and hornblende schist. (“Manhattan’s gneiss, but full of schist” goes the immortal line.) This is the good, hard stuff, the serious, no-nonsense rock, but everything has its weaknesses, and time grinds it all up, as in the mills of the gods.

Sandstone and Basalt

IMG_3353Kind of hot for blogging, wot? Let’s take a dip in the North Sea: this is Greymare Rock, also known as Saddle Rock and the Whale’s Belly, just south of Dunstanburgh Castle. It’s made up of buckled layers of sandstone, the same sandstone used to build the nearby castle. IMG_3345 The castle itself, a ruin now, was built on an exposed ridge of basaltic Whinn Sill, a natural wall to the sea. Brilliant budgeting, that. IMG_3351The pale discoloration in the far distance on the cliff is guano from birds. The cliff hosts a nesting colony of Kittiwakes.

Oh, Schist!

schistI never get tired of quoting that guy in the Times who wrote succinctly that “Manhattan is gneiss, but full of schist.” The bare bones of the little island of Mannahatta are exposed on the upper, upper west side where a ridge of mica schist, the famed Manhattan schist, rises over the flatlands of Harlem. The pictured flank is in St. Nicholas Park, at 137th and Saint Nicholas Ave. The neo-Gothic buildings of CCNY loom above.

That sure looks like rock varnish to me, something more famous in the southwest. Also going on here is sheeting, or contour weathering, a type of rock exfoliation due to the chisel of time and weather.ArtemisiaAnd in a little hollow, a patch of Mugwort (Artemisia)! The stuff grows anywhere. Its roots, like other plants punching into rock, will assist in the weathering away of this rock face over time.

Ice-dozer

striationsThe buck stopped here in what is now Brooklyn; indeed, the buck of glacial ice made Brooklyn and the rest of Long Island, depositing the rubble of rock and soil it had scraped forward until it stopped and retreated and left the jumble behind. Two pulses of glacial activity formed Long Island, leaving ridges that extend out through the North and South Forks; these are called terminal moraines. In Brooklyn the later moraine sort of smeared out the earlier one, so we’ve only one: it has a name — Harbor Hill, capped by Green-Wood and part of Prospect — known to few but geologists, and is best marked by neighborhoods with ridge, heights, hill, and slope as part of their name. Runoff of smaller particles from the moraines made the flatlands to the south, the outwash plain. Rising seawaters then sculpted the Island’s outline.

All this to say that you have to go to Central Park and the Bronx to find glacial striations, the grooves ground into exposed rock by the gritty underside of the glaciers. Here’s a patch of schist in the New York Botanical Garden’s forest. The groves run NW-SE, the direction of the ice. 10,000 or so years of erosion have softened them a bit.

Two Monhegan Cairns

cairn1Lobster Cove: sharply-edged fragments of igneous gabbro. cairn2Pebble Beach: sea-smoothed granite “pebbles”.


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