Posts Tagged 'Geology'

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.


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”.

Hello, Maine

Portland HeadPortland Headmaine3Three views from Portland Head Lighthouse.Clarry HillClarry Hill’s blueberry fields. MonheganThe rocky coast of Monhegan.MonheganLooking towards White Head from Black Head, Monhegan.Monhegan


cairnNo rocky place should be left unhonored. The Hudson shore at Croton Point. IMG_3283A weathered piece of old brick, from the kilns that fed the metropolis down-river, contrasts nicely with a downed tree, so like rock itself.

Desert Varnish

trunksThe “varnish” here, looking a little like apparitional tree trunks, is made up of clay, iron and manganese oxides, and some organic material. And time. The darker it is, the more manganese, a mineral rare on the planet.

In some accessible areas, this thin layer can be chipped off to reveal the lighter rock beneath. We call the resulting designs petroglyphs (from the Greek words for “stone” and “to carve”) when made by human hands. Here are some of the ones at Newspaper Rock, which is actually made up of several boulders, in Petrified Forest National Park in Arizona — and is not to be confused with Newspaper Rock State Historical Monument in Utah:Newspaper RockNewspaper RockNewspaper RockNewspaper RockThese, whose meaning remains unknown, are thought to have been made by people farming in the Puerco River Valley some 600-2,000 years ago. The area, as stark and arid as it is (of course, sometimes such conditions are excellent for preservation), has evidence of people dating back 8,000 years.

There are a lot of designs here, and it is fun to try to guess what they are, and mean, but I think I like the circles most of all. What is fully, dare we say perfectly, circular in this world? Perhaps nothing at all, but Sun, Moon (periodically), the eye (both iris and pupil, but not in the case of all mammals) certainly look like they are. What else?


Cedar BreaksAt Cedar Breaks, a Ranger gave us a good mnemonic for the geological history of the Colorado Plateau: “Cedar Breaks is due for a change” with “due” initialing for deposition, uplift, and erosion. Ancient lake and sea beds heaved up and then slowly, differentially, whittled away….Cedar Breaks

Bryce CanyonBryce Canyon in the fog. Not actually a canyon, Bryce is, like Cedar Breaks, the dramatically eroding edge of a plateau; we were standing at about 8000 feet here looking down. BryceThe famous hoodoos are created by frost weathering, fracturing via the expansion of freezing water. The “giant staircase” of the Colorado Plateau descends from this highpoint, geologically the most recent, down into the Grand Canyon, which has rock a billion years old at its base.Grand CanyonA minor tributary of the Grand Canyon from Bright Angel Point.

Antelope CanyonUpper Antelope Canyon, a slot in the Navajo Sandstone cut out by water erosion. In Navajo, Antelope is named Tsé bighánílíní, the place where the water runs through the rocks. While we were there it started to rain, and you could see the water pouring down the smooth walls, uncannily showing the potential of flash floods to rip through the slot. All the red rock in these photos is from iron oxide; rust, essentially, and I’ve intensified it here to bring out the contrasts in these positively sensual surfaces.Antelope

Monument ValleyMonument Valley’s buttes are composed of three separate layers of rock, each eroding at a different rate. It started to rain while we were here, too; water poured off the top of these, especially the aptly named Rain God Mesa, as waterfalls, doing their slow work.Monument Valley

Canyon de ChellyThe inspiration for Jabba the Hutt? At Canyon de Chelly.

Tender Buttons

clay1These smooth, hard clay nodules are from Croton Point Park, formerly the location of a brick factory. They were sticking out of a large pile of less-clayey material, as if the surrounding had been eroded away by… river, rain, wind, all of the previous? The largest is the diameter of a quarter.clayThis is what the bottom of the nodules looks like, a little nubbin where they were attached to the larger mass — they came off with a twist — with a reddish stain suggestive of iron.

Any idea what these are called, if indeed they have a geological term?

Modeled on a volcanic piece — that’s redundant — of Iceland, the most textured rock in the house, to contrast with the ever-so-fine particles of clay.

Update: Thanks to Kevin (see comments) for the news: these are concretions. Vermont has a state park named after them, but I think these Croton Point examples are finer than the ones from that corner of Lake Champlain that I’ve seen pictures of, anyway. There are some wonderfully grotesque ones, too, known as “clay dogs.” The American Naturalist of September 1884 refers to “clay dogs, clay stones or clay concretions.” Wikipedia bemoans the loss of them at Croton Point after the place was turned into a park. But clearly not….

A further update: the Nature Center at Croton Point has an exhibit of “clay babies,” some of them aptly fetal.

Update: I got a postcard from my friend Sue:Cave6PearlsShe was with us at Croton and was reminded of these buttons when she saw these “toupie et perles des cavernes” at the the Cro-Magon-era cave Pech Merle. These are made of calcite polished by water.

Rock Slide

In May, a 10,000-ton piece of rock broke off the nearly vertical face of the Palisades in Alpine, NJ, and came crashing down. Yesterday evening, we cruised by in the Commodore’s boat.

It obviously hasn’t taken long for some plant life to return. The Palisades were preserved into a park in 1900 after being deforested and even mined for decades, so many of the trees here now are opportunistic and hardy. There are, for instance, many Paulownia. In some places, you wonder if there’s any soil at all, and marvel at the trees’ grip on the ruins of old slides.


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