Tuesday, May 30, 2017

Geology Through Literature - The Last of the Mohicans

The next up on my Geology Through Literature thread is The Last of the Mohicans by James Fenimore Cooper published in 1826. You can get my complete thoughts on the book/story over at my blog - The Remnant, but for here I will just go into the geological or basic scientific aspects that are brought up in the story.

There is only one instance of geology that I could find worth mentioning but it is a long one. 

Midway through Chapter 6

     A spectral-looking figure stalked from out of the darkness behind the scout, and seizing a blazing brand, held it toward the further extremity of their place of retreat. Alice uttered a faint shriek, and even Cora rose to her feet, as this appalling object moved into the light; but a single word from Heyward calmed them, with the assurance it was only their attendant, Chingachgook, who, lifting another blanket, discovered that the cavern had two outlets. Then, holding the brand, he crossed a deep, narrow chasm in the rocks which ran at right angles with the passage they were in, but which, unlike that, was open to the heavens, and entered another cave, answering to the description of the first, in every essential particular. 
     "Such old foxes as Chingachgook and myself are not often caught in a barrow with one hole," said Hawkeye, laughing; "you can easily see the cunning of the place - the rock is black limestone, which everybody knows is soft; it makes no uncomfortable pillow, where brush and pine wood is scarce; well, the fall was once a few yards below us, and I dare to say was, in its time, as regular and as handsome a sheet of water as any along the Hudson. But old age is a great injury to good looks, as these sweet young ladies have yet to l'arn! The place is sadly changed! These rocks are full of cracks, and in some places they are softer than at othersome, and the water has worked out deep hollows for itself, until it has fallen back, ay, some hundred feet, breaking here and wearing there, until the falls have neither shape nor consistency." 
     "In what part of them are we?" asked Heyward. 
     "Why, we are nigh the spot that Providence first placed them at, but where, it seems, they were too rebellious to stay. The rock proved softer on each side of us, and so they left the center of the river bare and dry, first working out these two little holes for us to hide in." 
     "We are then on an island!" 
     "Ay! there are the falls on two sides of us, and the river above and below. If you had daylight, it would be worth the trouble to step up on the height of this rock, and look at the perversity of the water. It falls by no rule at all, sometimes it leaps, sometimes it tumbles; there it skips; here it shoots; in one place 'tis white as snow, and in another 'tis green as grass; hereabouts, it pitches into deep hollows, that rumble and crush the 'arth; and thereaways, it ripples and sings like a brook, fashioning whirlpools and gullies in the old stone, as it 'twas no harder than trodden clay. The whole design of the river seems disconcerted. First it runs smoothly, as if meaning to go down the descent as things were ordered; then it angles about and faces the shores; nor are there places wanting where it looks backward, as if unwilling to leave the wilderness, to mingle with the salt. Ay, lady, the fine cobweb-looking cloth you wear at your throat is coarse, and like a fishnet, to little spots I can show you, where the river fabricates all sorts of images, as if having broke loose from order, it would try its hand at everything. And yet what does it amount to! After the water has been suffered so to have its will, for a time, like a headstrong man, it is gathered together by the hand that made it, and a few rods below you may see it all, flowing on steadily toward the sea, as was foreordained from the first foundation of the 'arth!" 
     While his auditors received a cheering assurance of the security of their place of concealment from this untutored description of Glenn's, they were much inclined to judge differently from Hawkeye, of its wild beauties. 

The location being described above is Glens Falls (spelled "Glenn's" in the story) on the Hudson River in current day New York. Although the story is fiction, the caves are based on an actual set of caves located on an island within the Hudson River at Glens Falls. The caves have been named in honor of James Fenimore Cooper for popularizing the caves and are now known as Cooper's Cave

Aerial view of the river showing the island, which currently has a bridge laying on top of it (I'm pretty sure the bridge did not exist in the time of the story).

An excellent image of Cooper's Cave (center crack) from the Wireman blog

Given the extraordinary detail that Cooper gives into describing cave formation in the story, I feel I really don't have much to add in that regards. It sounds good and it's pretty spot on with how it actually likely occurred.

The black limestone in question is a fossiliferous unit called the Glens Falls Limestone, and it is part of the Trenton Group from the Late Ordovician (Mohawkian).

Here is some text on the area from the Historic American Engineering Record on Glens Falls Dam:

The Glens Falls Dam is situated at the head of Glens Falls on the Hudson River, a natural rock descent over Glens Falls limestone. The top of the falls is at an elevation of approximately 256 feet. A descent of 36 feet of the limestone bedrock occurs over a horizontal distance of 200 to 320 feet, culminating in a pool at the bottom of the falls with an elevation of 220 feet. Two channels have developed at the falls. The secondary channel lies toward the south side of the Hudson River. The main channel is closer to the north side of the river. Since the main channel carries more water than the secondary channel, it has eroded upstream at a faster rate. Therefore, the falls begin farther upstream on the north side of the river than on the south side. This difference in erosion rates has determined the position and shape of the Glens Falls Dam. Instead of being a single straight-line structure perpendicular to the water's flow, and spanning the river from bank to bank, the Glens Falls Dam is constructed in three sections that follow the arc of the highest part of the bedrock at the top of the falls.

The Glens Falls Limestone can be subdivided into the into a lower unit, the Larrabee Limestone, and an upper unit, the Shoreham limestone. The limestone itself is abundantly fossiliferous and formed in the deeper-shelf  (Garver, 1995).

Here is a bit of the basic description of the unit from Geolex:
Named the Glens Falls limestone of the Trenton group for Glens Falls, Warren Co., eastern NY. Consists of thin layers of very fossiliferous limestone with shale intercalations near top and a 2 inch conglomerate layer at base. Contains ripple marks and other signs of shallow water conditions. Basal unit of Trenton group. Thickness is 17 feet. Overlies the Tribes Hill limestone or the Amsterdam limestone and underlies the Canajoharie shale. Fossils indicate that the Glens Falls is of Middle Ordovician age.

And a little bit of more in depth information from the US Geological Survey.
Middle Ordovician New York and Vermont and Ontario Canada  
G.M., Kay, 1937, Geol. Soc. America Bull., v. 48, no. 2, p. 264-267. 
Member of Sherman Fall formation. Constitutes zone of Cryptolithus tesselatus Green, the limestones of lowest Sherman Fall age. In type region, beds comprise upper Glens Falls limestone, overlie lower Glens Falls Larrabee member of Hull age, and underlie Canajoharie shale of later Sherman Fall age. Beds consistently contain Cryptolithus tessclatus, which is limited to the member and Prasopora orientalis Ulrich and Trematis terminalis Emmons. This zone persists in the Sherman Fall northwestward to Lennox and Addington County, Ont., in the equivalent beds of northern Lake Champlain, and northeastward to city of Quebec. In type section, lower 36 feet of member is exposed. Along Mohawk Valley, member is composed of 15 to 25 feet of dark-gray calcareous claystones and shales that contrast with subjacent Larrabee member, and are succeeded abruptly by Canajoharie shale. Member has exposed thickness of 30 feet north of McBrides Bay, South Hero Township, Grande Isle County, VT, with a metabentonite 11 feet from base. Overlying beds are Cumberland Head shaly limestone and Stony Point shale, both of later Sherman Fall age. In New York underlies Denmark member (new).

And there you have it. Another piece of evidence that a piece of historical literature can turn a geological based location into a landmark.

No comments:

Post a Comment

Due to the large number of spam comment (i.e. pretty much all of them). I have turned off commenting. If you have any constructive comments you would like to make please direct them at my Twitter handle @Jazinator. I apologize for the inconvenience.

Note: Only a member of this blog may post a comment.