Tuesday, November 14, 2023

Geology through Literature - Walden

Using Walden by Henry David Thoreau

Science is not a new invention. People have been performing science for many thousands of years. Often they build on the research of those before them and sometimes they start from scratch. The purpose of this project is to use a scientific study from the 19th century to produce a current contour map of lake depth.

The study being described is in Walden by Henry David Thoreau, written before 1854. The book is typically considered “philosophical” literature but in this case he performs the basis of science. He identified a problem, determined how to solve the problem, and then executed the research. Below are a series of assignment questions and directions, however I have also included my answers and some additional information. There is a link at the bottom to the assignment itself without the answers intermingled. 

His Problem:
Often people would describe the depth of Walden Pond as bottomless. He wished to prove them wrong and determine the actual depth.

His Method
 To determine the actual depth of the lake he used the simple method of a rock and string.

His Solution:
That’s where you come in.

Project Directions
1. Read the “The Pond in Winter” chapter of Walden by Thoreau. For reference the important excerpts from the chapter are provided here:
"As I was desirous to recover the long lost bottom of Walden Pond, I surveyed it carefully, before the ice broke up, early in '46, with compass and chain and sounding line. There have been many stories told about the bottom, or rather no bottom, of this pond, which certainly had no foundation for themselves.... Many believed that Walden reached quite through to the other side of the globe.... But I can assure my readers that Walden has a reasonably tight bottom at a not unreasonable, though at an unusual, depth. I fathomed it easily with a cod-line and a stone weighing about a pound and a half, and could tell accurately when the stone left the bottom, by having to pull so much harder before the water got underneath to help me. The greatest depth was exactly one hundred and two feet; to which may be added the five feet which it has risen since, making one hundred and seven feet." 
"...this one, which is so unusually deep for its area, appears in a vertical section through its centre not deeper than a shallow plate." 
"As I sounded through the ice I could determine the shape of the bottom with greater accuracy than is possible in surveying harbors which do not freeze over, and I was surprised at its general regularity. In the deepest part there are several acres more level than almost any field which is exposed to the sun, wind, and plow. In one instance, on a line arbitrarily chosen, the depth did not vary more than one foot in thirty rods; and generally, near the middle, I could calculate the variation for each one hundred feet in any direction beforehand within three or hour inches." 
"... Having noticed that the number indicating the greatest depth was apparently in the centre of the map, I laid a rule on the map lengthwise, and then breadthwise, and found, to my surprise, that the line of greatest length intersected the line of greatest breadth exactly at the point of greatest depth, notwithstanding that the middle is so nearly level, the outline of the pond far from regular, and the extreme length and breadth were got by measuring into the coves...." 
"Of five coves, three, or all which had been sounded, were observed to have a bar quite across their mouths and deeper water within, so that the bay tended to be an expansion of water within the land not only horizontally but vertically, and to for a basin or independent pond, the direction of the two capes showing the course of the bar.... In proportion as the mouth of the cove was wider compared with its length, the water over the bar was deeper compared with that in the basin."

2. Write down all important sentences and phrases that have to do with the depth and shape of the pond.

3. Summarize these into only the important points (like the location and depth of the deepest point).
  • The deepest point of the lake is 102-107 feet deep and is located at the intersection of the greatest breadth and the greatest width lines.
  • There are sandbars that surround the coves creating mini lakes.
  • The pond is regular, meaning that the contours are evenly spaced apart.
  • The base of the pond is relatively flat.
4. Use one of the outlines of Walden Pond provided to start and outline the important features (deepest point, sand bars, etc.) in pencil.

In order to provide a map that is usable for this exercise, I first took an aerial image of Walden Pond. 

This was then traced and the aerial image was removed with the approximate location of Thoreau's cabin added for context.

5. Make a contour depth map (bathymetric map) with 20ft contours. The shore of the lake will be your 0 contour (provided). Then erase all of the mistakes and non-important items on the map so you just have a finalized contour map left.

Following these directions, as well as directions on how to make a contour (bathymetric) map, people should be able to make a fairly accurate Walden Pond bathymetry map.

And then students can compare their results with a professional map result called Hydrology and Trophic Ecology of Walden Pond, Concord, Massachusetts by Paul J. Friesz and John A. Colman, 2001 seen below:

You can also get a zip file with a complete set of directions as well as a step by step solution over at my site - http://www.dinojim.com/Geology/GeoEducation/GeologyThroughLiterature.html

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