Sunday, November 26, 2023

Go and See

In the historical development of geology, there were conflicting ideas about the formation of crystalline rocks (the rocks we today refer to as igneous or metamorphic). In the late 1700s, an influential German mineralogist named Abraham Gottlob Werner: (1749-1817) argued that crystalline sediments chemically precipitated from an initial universal ocean early in Earth’s history. While he published little, and frail health precluded traveling beyond his local area, Werner was a very influential professor at the Freiberg Mining Academy where he infected students with an almost disciple-like fervor. This concept of rocks precipitating from ocean water became known as Neptunism.

Another famous early geologist was James Hutton (1726 –1797), a wealthy Scottish polymath who dabbled in medicine, chemical manufacturing, agriculture, and geology. Often called the “Father of Modern Geology”, he wrote Theory of the Earth in 1788 where he argued for the principle of uniformitarianism – the idea that the same processes that operate on Earth today were the same that have operated throughout Earth history. He also advocated that crystalline rocks formed gradually over time from the cooling and crystallization of molten rock or magma. This idea was in direct opposition to Werner’s idea of Neptunism and came to be known as Plutonism setting the stage for a vociferous debate between these two ideas in geology in the late 1700s.

Meanwhile, in France, a geologist named Nicolas Desmarest (1725-1815) was studying and mapping rocks in Auvergne, a mountainous area in central France. While his passion was geology, Desmarest supported himself with a government job as Inspector General and Director of Manufactures which enabled him to travel around the country, often on foot so he could examine the local rocks.

In Auvergne, Desmarest found columnar basalt lava flows which he was able to trace back to volcanic-like craters in the region. It was clear that the basalt lava was erupted from these extinct craters. He presented a geologic map of the area to the Paris Academy of Sciences which was later published in 1777. His results clearly supported the ideas of Plutonism.

Map of the Auvergne lava flows by Desmarest

Columnar basalt lava flows in the the Auvergne region

Extinct volcanoes in the Auvergne region

When Desmarest was approached by those who wished to debate Neptunism vs. Plutonism, he was reputed to have said “Go and see.” This quote, which appears in most introductory textbooks in historical geology, strongly resonates with modern geologists. Theorizing about the Earth is all well and good, but if your ideas don’t stand up to field checking, there’s little point debating it.

That’s why we have field trips in geology. It’s one thing to tell students that the Hudson Valley was once covered with a warm, shallow sea – a fact that they’ll dutifully write in their notebooks without questioning. It’s another to take them to a limestone rock outcrop on the side of a local highway and show them the abundant marine invertebrate fossils in the rock. Want to learn about the geology of the Hudson Valley? Go and see – look at the rocks and let them speak to you.

Tuesday, March 22, 2022

Cranberry Lake Preserve

 Went for a walk last week to the Cranberry Lake Preserve in Valhalla, Westchester County. It's a 190 acre biodiversity preserve with some hiking trails (a horribly-designed set of hiking trails, by the way, with the same color blaze used for completely different trails - without a map in hand you'd easily go astray). Of course Cranberry Lake Preserve surrounds, not surprisingly, the pretty Cranberry Lake.

Of interest to us, however, is an abandoned quarry built to supply stone for the nearby Kensico Dam - part of the New York City water supply system - which was completed in 1917.

Walking the trails from the parking lot reveals what appears to be the Fordham Gneiss (the g is silent and gneiss is pronounced "nice"). Fordham Gneiss is a high-grade metamorphic rock formed during the Grenville orogeny (an orogeny is a mountain building event) a bit over a billion years ago. It was formed from high temperature and pressure alteration of sediments and volcanic material (probably associated with a volcanic island arc) caught up in a massive collision between proto-North American (called Laurentia) and, we think, a block of crust called Amazonia. This collision formed Himalayan-scale mountains right here on what was an ancient supercontinent called Rodinia (hundreds of millions of years prior to the later supercontinent of Pangaea).

This gneiss, that we walk on today, was once miles below the surface of those ancient Grenville Mountains only exposed today after a billion years of erosion!

The quarry that supplied rock for the Kensico Dam is on the other side of the lake from the parking area. A few of the pits were filled with water and supposedly once used as swimming holes by the locals.

The rock they were quarrying here was not the Fordham Gneiss, however, it was the Yonkers Gneiss. You might think gneiss is gneiss, but that's not the case. The Yonkers Gneiss is what's called a granitic gneiss meaning it formed from the metamorphism of the igneous rock granite (as opposed to the Fordham Gneiss which formed from sediments and volcanic material). The Yonkers Gneiss is quite attractive (even more so when cut and polished) and was valued as a building stone (in the building trade, it's sometimes called the Yonkers granite but that's geologically incorrect).

While the Yonkers Gneiss has long been thought to be associated with the Fordham Gneiss, more recent data suggests that it's much younger. Instead of being slightly over a billion years old, it seems to be closer to 560 million years ago and associated with the rifting apart of the supercontinent of Rodinia (A-type granitoid associated with continental rifting for the geologically inclined).

Even if you don't care about the geological mysteries here, it is a nice hike. In mid-March it's easier to see the geology without all the green stuff, mosquitos were still absent, and we were serenaded by a chorus of wood frogs and spring peepers.

Sunday, March 13, 2022

Geology is where you find it...

So I went to the American Museum of Natural History in Manhattan a few weeks ago to see their relatively new Hall of Gems and Minerals (highly recommended, by the way). No, I'm not going to post a picture of some magnificent mineral (of which there were many) but rather a picture of a piece of old artillery on display that caught my eye.

This is a Parrott rifle shell. A Parrott rifle was what most of us would call a cannon (see below). They're named after Robert Parker Parrott, an ordnance officer in the U.S. Army who inspected canons manufactured at the West Point Foundry in Cold Spring, New York. In 1836 he was hired by the foundry to solve a problem - how to make a cast iron rifled cannon barrel - one with spiraling grooves cut into the inside of the barrel to improve the performance of the canon. Previous attempts with cast iron had a disconcerting habit of blowing up when used. The result was the Parrott rifle where a  band of wrought iron over the barrel (mostly) kept it from blowing apart during battle.. Parrott rifles were manufactured by the West Point Foundry between 1860 and 1889 and extensively used by the Union Army during the U.S. Civil War.

According to the display sign for the above Parrott shell on display in the Museum, it was found off the coast of Fort Hickory in North Carolina.

Unfortunately, neither Google or Google Maps returns anything called "Fort Hickory" in North Carolina (or anywhere else on the eastern coastline). I have a picture of the label so I know I didn't misread it making it a complete mystery to me. If anyone knows where Fort Hickory is, let me know.

As a side note for trivia nerds, the West Point Foundry was where Jules Verne, in his 1867 novel From the Earth to the Moon, had the Columbiad manufactured - the cannon that launched his spacecraft to the Moon.

… the iron ore, molten in the great furnaces of Coldspring, and brought into contact with coal and silicium heated to a high temperature, was carburized and transformed into cast iron.

Anyway, so what's the geology connection? This shell, supposedly found off the Carolina coast, was forged from Hudson Highlands iron ore -specifically the mineral magnetite (Fe3O4). I don't know why, but I find this kind of stuff interesting.

A mineral that formed a billion years ago in an ancient mountain range formed when continents collided into a supercontinent called Rodinia just happened to be in the proximity of a strategically important river to the early United States where it was exploited for making armaments in a famous foundry associated with the military academy at West Point (at a strategic location along the Hudson for the Revolutionary War). These cannons and shells were used in the Civil War and a chunk of this iron landed in the Atlantic Ocean off the southern coast where it was found and eventually brought back to its native New York for display.

Sunday, March 6, 2022

Armored Mud Balls

This week's post takes us a little outside of the Hudson Valley and over to the Connecticut Valley in Massachusetts. Specifically, to a town named Greenfield right on the Connecticut River and only about 12 miles or so south of the Vermont border. At Greenfield, the Connecticut River tumbled over Turner Falls where some neat rocks are exposed from the early Jurassic Period of geologic time.

Now we've all heard of the Jurassic because of the Jurassic Park movie franchise, but I wasn't visiting Greenfield to see dinosaur remains (although there are some trackways in the area) but rather to look at lowly mud balls. These aren't ordinary mud balls, however, but quite unique armored mud balls fossilized in those Jurassic-aged rocks.

During the Jurassic Period, the supercontinent of Pangaea was rifting apart. As Africa rifted away from North America to form the Atlantic Ocean, the crust stretched a bit further inland as well forming what are known as continental rifts (similar to the East African Rift today). The Connecticut Valley is one such continental rift from that period of time.

Below is a figure showing the geology of this area.

Now I could write a number of blog posts about the geology shown here, everything from basalt lavas pouring from deep faults to dinosaur trackway mysteries, to massive post-glacial lakes but today's topic are the mud balls.

Picture yourself standing in north-central Massachusetts some 200 million years ago. You're on the shore of a large shallow lake. It's a hot, arid climate and, off in the distance, you see some lavas flows erupting from a fissure at the base of some mountains. You're cautious, since two-legged therapod (meat-eating dinosaurs) are scouting around the area.

One afternoon, a distant thunderstorm drops some rain in the mountains. As water runs across the baked landscape, chunks of dried clay mud fall into the dry washes that are now filling with runoff from the rains and becoming streams. The streams flow down toward the lake, rounding off the chunks off mud as they tumble along. Becoming sticky, these mud balls pick up pebbles in the stream bed forming an armored coating around them. Finally, these armored mud balls come to rest in the sandy sediments of the lake bed (today known as the Turners Falls Sandstone).

A neat little story and not something that's normally preserved. Apparently, these types of mud balls are only known from about 10 places around the world. The best place to see them today is on the campus of Greenfield Community College which has a very neat Geology Path showing rocks of the area and some great examples of these mud balls.

Here are some pictures...

Worth a stop if you're ever in the area and have an interest in unusual geologic features!