Old stone walls record history of Earth’s magnetic wanderings

From AGU

27 February 2019 

Posted by llester

By Liza Lester

https://youtu.be/G3OTG3_ih3c

Under the forests of New York and New England, a hidden tracery of tumbledown stone walls marks the boundaries of early American farms, long abandoned for city jobs and less stony pastures in the West.

These monuments of an agrarian past also mark past locations of Earth’s itinerant magnetic north pole, a record leveraged by geochemist and local history buff John Delano to reconstruct a history of our planet’s magnetic field in eastern North America in a new study published in AGU’s Journal of Geophysical Research: Solid Earth.

“As a little kid growing up in the country in New Hampshire, I was fascinated with stone walls that were in the middle of the woods. Who made them? Why?” said Delano, an emeritus professor at the State University of New York at Albany. “Post-retirement, I had the time to pick that study up and wondered, now with a different skill set: What do they remember? What are they telling us?”

On a visit to his local historical society, Delano had a eureka moment while looking at map from 1790. From the jumble of stone walls on his present-day map, a grid pattern emerged that looked much like the property boundaries of the many 100-acre farms in the late 18th century township.

Delano got his hands on hundreds of original, 18th and 19th century surveys from the New York State archives and overlaid modern aerial images of the stone walls to find the walls that marked the old property boundaries. Using GPS, he measured the walls’ present-day bearings with respect to True North and compared them to compass bearings for the boundary lines, recorded by the 18th and 19th century surveyors.

An old stone wall marks a boundary of a long-abandoned farm near Grafton, New York, once part of the colonial Manor of Rensselaerwyck. A section of the 700,000-acre manor west of the Hudson River was was surveyed for rental allotments in 1787. Credit: John Delano

An old stone wall marks a boundary of a long-abandoned farm near Grafton, New York, once part of the colonial Manor of Rensselaerwyck. A section of the 700,000-acre manor west of the Hudson River was was surveyed for rental allotments in 1787.
Credit: John Delano

The discrepancy between Delano’s measurements and the historical compass readings is not the error of the early surveyors, but the magnetic declination at the time of the survey. The difference between True North and magnetic north shifts over time due to changes in Earth’s outer core.

Modeling the wanderings of Earth’s magnetic field can provide clues to the enigmatic motions deep in the Earth that generate it.

“Some geophysicists who are trying to model these complex motions of fluids are helped in their analysis by having a very accurate record of how the declination has moved over time,” Delano said.

Wayward poles

north-polar-wander-1590-2020-50percent

Historical magnetic declination 1590-2020. Credit: NOAA National Centers for Environmental Information

Geologists and navigators have long known that the north pole found by magnetic compass is not the same as True North, the point at 90 degrees N latitude that sits on the axis of Earth’s spin. The angle of deviation is called magnetic declination, and its degree varies depending on where the compass holder stands on the globe.

This difference matters not just for bushwhackers making their way without a GPS device, but for military and commercial aircraft, ships, submarines and even smartphones, which still use Earth’s magnetic field for orienteering.

Declination information must be updated frequently, because, unlike True North and South, Earth’s magnetic north and south poles are not fixed points. They move by tens of meters (yards) every day at erratic rates and directions. Motions in the fluid, molten metals that surround Earth’s core are believed to generate the magnetic field, although the deep layers of the planet cannot be observed directly. The dynamics of the system are not well understood.

Scientists can only model future positions of the magnetic north and south poles a few years into the future. The magnetic north pole has moved so quickly in the last few years, at 50 kilometers (30 miles) per year, the U.S. National Geophysical Data Center and the British Geological Survey released an early update to the joint World Magnetic Model’s 5-year cycle earlier this month.

Finding north with old stone walls

Historical data on the movements of the magnetic north pole provide clues to the behavior of Earth’s dynamo. Previous research groups have modeled magnetic declination from 1590 to today, based primarily on extensive sailing ships’ log records of magnetic north and the position of the North Star.

“That’s magnificent work, enormously tedious, with great detail and commitment on their part to do it, but they had little land-based data. And that’s where my work came in,” Delano said. “Using land-based methods, an entirely different approach, how would it match up, or not, with the current geophysical model?”

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60 thoughts on “Old stone walls record history of Earth’s magnetic wanderings

    • I doubt that they do. However, I was a Air Force navigator in the 1960s and 1970s and crossed the Pacific Ocean many time, navigating the old fashioned way with celestial and a N-1 magnetic compass. It didn’t take long for me to notice that the declination values printed on the charts invariably resulted in me being off course during transit between Hawaii and Guam — and always in the same direction. I took to noting the deviation. At its extreme it was four degrees off what the charts were giving me.

  1. That’s actually brilliant. I wouldn’t have thought of it. Surveying wasn’t the same then. My assumption that everybody used true north is wrong.

    Everything in a particular area of Southern Ontario lines up in stupid directions. Why? It seems that the early surveyors thought there were more important things than north and south. link

    • comieBob
      And there are some places, like Magnet Cove (Ark), where the presence of large iron-ore bodies make compass readings unreliable. I’m sure that you have some in Ontario.

    • general (surveying) rule for reconciling old legals & surveys,

      1) Bearing first
      2) Distance second

      Because it is assumed that the origin (occupants or surveyors) had better access/ability to direction than distance measurements.

      I never really agreed with this as a rule, but it is the way it is (although it very very seldom comes into play).

      The moving “north” is a good reason to get rid of the general rule & replace with general guideline.

    • Not markers, walls. Not a surveyor, but a lot of early property boundaries were essentially marked by N-S and E-W boundaries, subject to local adjustments such as creeks, rivers, mountains, etc. studying the orientations of old fieldstone walls (especially in a statically significant number) can give a good estimate of the orientation of magnetic north at the time the walls were built (or at least that’s my understanding of the article).

    • “As a little kid growing up in the country in New Hampshire, I was fascinated with stone walls that were in the middle of the woods.”

      Just goes to show wherever there’s enough water, cleared land left unmowed will revert to forest. I don’t think people who donate to plant trees thinking they are helping the environment understand this.

    • Nothing.
      If you read the article it says that existence of these walls was what got the author interested in old surveys. These surveys that were then used to determine magnetic north at the time of the surveys.

    • There is a term called magnetic variation. It defines the difference between magnetic North and true North in angular measure (degrees). Over time, magnetic variation varies because the magnetic poles wander while the true (aka geographic) pole remains the same.
      Those old surveyors used magnetic compasses to determine their orientation; all those directions are oriented to the magnetic north that existed when the surveyors laid our their plats of land. Measuring the orientation of the walls with respect to current magnetic north (and probably true north too) researchers are able to deduce the magnetic variation of those sites several hundred years ago.

    • Al,

      legal descriptions were based on “north” as the basis of bearing.

      the author assumes (or has determined) that property lines were established by compass and “north” (magnetic north at the time).

      the stone walls were built along the established property lines.

      the difference between today’s magnetic north the wall orientation shows the net movement of magnetic north over the time ….

      (although if true north was determined through solar observation, or some other means, then the premise is flawed. maybe the old surveys specifically call out magnetic north.)

  2. In the 1970s we noted the deviation of late 1800s and subsequent farm fences in dry inland Australia.They were wood post and wire, following surveyed tenement boundaries. Replacement usually took the same path, so the deviation could reconstruct the then magnetic field vector. Geoff

  3. Side note, a lot of boundaries (e.g., southern boundary of North Carolina, Mason-dixon line, etc.) are kinda weird in parts because even surveyors make mistakes.

    • There’s a long ongoing dispute between Georgia and Tennessee regarding their border. Georgia says it’s further north because it wants water from the Tennessee River for Atlanta.

      In 1818, a surveying mistake put the official Georgia-Tennessee border in the wrong place, about 1 mile south of the 35th parallel. Georgia, backed by modern technology, maintains its actual territory extends now about 60 square miles into Tennessee. But all Georgia really wants is for its border to reach one quarter-mile into the Tennessee River.

      https://www.cbsnews.com/news/water-wars-tennessee-georgia-locked-in-battle-over-waterway-access/

    • Currency was somewhat rare and heavy in early North America (gold and silver coins – paper was distrusted) so many surveyors were paid in Rum and Whiskey. Explains a lot of weird early surveying lines.

    • There has always been some discussion about why a surveyor with good skills and results suddenly seems to have more error/mistakes.

      One guess was that as they made their way west they began carrying guns, which interfered with compass readings.

      My guess is that it consistently has to do with the experience of a people that work for the surveyor, and a new crew doesn’t carry the surveyor as well.

  4. Somebody posted a link a while back about the mishap at Donner Pass. That article described how settlers traveling to their homesteads in the midwest plains tied a rag to a wagon wheel spoke, and the children would take turns counting wheel revolutions so they could find their land. I can imagine that caused all kinds of boundary problems.

  5. One of the greatest American Poems:

    Mending Wall by Robert Frost (1874–1963)

    SOMETHING there is that doesn’t love a wall,
    That sends the frozen-ground-swell under it,
    And spills the upper boulders in the sun;
    And makes gaps even two can pass abreast.
    The work of hunters is another thing:
    I have come after them and made repair
    Where they have left not one stone on stone,
    But they would have the rabbit out of hiding,
    To please the yelping dogs. The gaps I mean,
    No one has seen them made or heard them made,
    But at spring mending-time we find them there.
    I let my neighbor know beyond the hill;
    And on a day we meet to walk the line
    And set the wall between us once again.
    We keep the wall between us as we go.
    To each the boulders that have fallen to each.
    And some are loaves and some so nearly balls
    We have to use a spell to make them balance:
    “Stay where you are until our backs are turned!”
    We wear our fingers rough with handling them.
    Oh, just another kind of outdoor game,
    One on a side. It comes to little more:
    He is all pine and I am apple-orchard.
    My apple trees will never get across
    And eat the cones under his pines, I tell him.
    He only says, “Good fences make good neighbors.”
    Spring is the mischief in me, and I wonder
    If I could put a notion in his head:
    “Why do they make good neighbors? Isn’t it
    Where there are cows? But here there are no cows.
    Before I built a wall I’d ask to know
    What I was walling in or walling out,
    And to whom I was like to give offence.
    Something there is that doesn’t love a wall,
    That wants it down!” I could say “Elves” to him,
    But it’s not elves exactly, and I’d rather
    He said it for himself. I see him there,
    Bringing a stone grasped firmly by the top
    In each hand, like an old-stone savage armed.
    He moves in darkness as it seems to me,
    Not of woods only and the shade of trees.
    He will not go behind his father’s saying,
    And he likes having thought of it so well
    He says again, “Good fences make good neighbors.”

    =====================================================
    The Frost poem is very deep. It can be read against itself. On the surface it holds that “Something there is that doesn’t love a wall,” a protest against being walled out. And that is the way I read it in my salad days when I was green.

    But his neighbor’s point is made in the action. The existence and repair of the wall have forced these flinty old New Englanders to to work together. Good neighbors work together to solve problems. The wall has made them good neighbors by making them work together. The wall does not wall them out, it brings them together.

    The ritual is not rational. “Isn’t it Where there are cows? But here there are no cows.” But it is a ritual “Oh, just another kind of out-door game” And it works as rituals do.

  6. Not sure why remote rural walls would be oriented magnetically north/south by use of some reasonably sizable instrumental compass anyway, when a ready at hand simple plumb-vertical stake driven into the ground will provide forenoon and afternoon equal length pole-top sun shadow points on level ground which provide a fine true east/west geographic line, and of course is in turn the key to a true perpendicular north/south direction as well.

    • The problem with using a stake shadow is the time factor. It would involve tying up 1 day per directional measurement, even if the result would be accurate enough to be useful. Imagine setting up that stake about 7 hours before local noon, then marking the tip of the shadow every minute for the rest of the day, until about 7 hours after local noon. Why start so early and mark so many points? Because you don’t have a good enough timepiece to tell when local noon will be within 1/2 hour, so start early to be safe. You also won’t know which way is true south until the middle of the day has passed. Once shadows begin lengthening, the shortest shadow can be used to determined true south.
      Later, the shadow formed 6 hours after the shortest shadow was formed can be used to determine true west, just as the shadow formed 6 hours before local noon marks due east. Of course you wouldn’t know when those two times were because you don’t have a good timepiece. You will have to make several careful measurements of the distance between the base of your stake and the various markers for the shadow tips to find the right markers. This isn’t easy because the the stake shadow is over 20 feet long at 6 am and 6 pm. If the ground isn’t perfectly level the shadow lengths will be off by a few inches even at noon, and several feet off when trying to determine the east-west line. The land in the pic appeared to slope so that would definitely be an issue.
      OK, try a better way: set up a large metal disk in the bed of a wagon with some method of leveling the disk. Affix a vertical rod to the center of the disk. Mark off degrees around the perimeter of the disk. Move the wagon out to a corner point shortly before local noon. Once the shortest shadow is determined, turn the 180 degree mark on the disk to align with that shadow’s direction. Now any bearing can be sighted using the degree markings on the disk if a sight hole is bored through the vertical rod and the rod can be turned independently from the disk.
      You are still limited to determining one bearing line per day, because you can only be in one place at noon. With a compass you can make multiple bearing sightings per day. You just have to base your system on magnetic north instead of true north, unless you have previously determined your local magnetic declination.

      • Thanks for this interesting post. You can also pound a stake into the earth pointing at the sun, so there is no shadow. As the sun moves, the shadow will point East.

      • Whoa, Steve. You don’t need to a clock and you don’t need to take multiple AM and PM shadow tip readings. Mark a pole-top shadow location and at the same time its distance from the pole with a length of string when the AM pole shadow is several feet long; then mark a PM pole-top shadow point that is the same shadow and string length from pole. Done. Those become the 2 points which determine a true east/west line on a flat surface.

        So don’t leave it to a runaway imagination, try it yourself any sunny day with a vertical pole, string, and some way to mark the 2 points. And when you do you will also note your frustration trying to determine south with any precision by when the noon shadow is at its shortest, as the shadow length around noon doesn’t change much with the passage of a considerable time.

        And not so fast about that vertical pole shadow reliably running true west 6 hours before and east 6 hours after local noon. That is only true on the equinox dates (Mar.21, Sept.23). During the spring/summer months between those dates the 6 hour from noon shadow directions will slant north of your expectation. And in any case to the winter side of those dates you won’t even be granted sunny 6 hour intervals either side of noon to toy with.

        • Doc Chuck:
          ” Mark a pole-top shadow location and at the same time its distance from the pole with a length of string when the AM pole shadow is several feet long; then mark a PM pole-top shadow point that is the same shadow and string length from pole. Done. Those become the 2 points which determine a true east/west line on a flat surface.”

          You again say “Mark a pole-top shadow” as if any AM shadow will point due east, when only one shadow line will. Yes, as you say, the due east shadow will have a complementary shadow pointing due west in the afternoon (The east-west shadow pair will be the only pair of opposing shadows that are equal length.), but you won’t know which of the AM shadows it will be until you have the afternoon set of shadows marked to compare with.

          “vertical pole shadow reliably running true west 6 hours before and east 6 hours after local noon. That is only true on the equinox dates (Mar.21, Sept.23). During the spring/summer months between those dates the 6 hour from noon shadow directions will slant north of your expectation”

          What you say about the 6 hour from noon shadow laying north of east or west lines (in the Northern hemisphere) is a common misperception, because local noon is not in use anymore, and because of the use of daylight savings time.
          For a few years my work involved locating geosynchronous satellites in the sky by use of a compass, and also noting what time of day the sun aligned with a satellite’s meridian. During the year the sun will appear higher or lower than a given satellite, but aligns with the satellite’s meridian at the same time, with a 1 hour shift due to daylight saving time. (This is why I specified 6 hours before & after local noon.) I can assure you the sun is indeed due west at 6 hours after local noon, just higher or lower depending upon the time of year. Midwinter it may be so low as to be below the horizon, but it is still due west.
          1 other confusion factor is due to Earth’s elliptical orbit giving the Earth a varying orbital velocity. The effect is the sun runs fast or slow at different times of the year. During the months of June through August in New England the sun varies between 2 minutes fast and 6 minutes slow. This matters if you are using a modern timepiece, but was irrelevant to our example back in the 1700’s.

          PS The sun is 7-8 minutes slow or fast on the equinoxes, & 14-16 minutes slow or fast during the winter, but no one would be out surveying then.

          SR

        • Doc Chuck, As I was posting the above reply, I realized you must have meant mark out AM and PM shadows that form a V with equal length sides, then connect the end points to make an east-west line. This method would allow you to make only 1 AM shadow mark, but would require you to mark the shadow tip for several minutes in the afternoon to catch the shadow at the right length. Thus you would still tie up one day per corner point finding the next bearing line.

          My main point in my first reply was to point out how tedious and time consuming a sun-shadow surveying method would be. The difficulty with finding true south is yet another troublesome issue, thought it could be managed by dissecting between two equal length shadows marked before and after noon when shadow lengths were more discernible. This dissecting line would be a useful check since it should align with a line dissecting the V used to find the east-west line.

          OK, this horse has been beat to death at least twice.

          SR

          • Steve, Now that I’ve gotten across the intended word picture of the V shape between the north slanted AM and PM vertical stick shadows which are of equal length to their tips (the marked location of which then in turn serve as the points for a true east/west line), I’m confident this horse will carry us a few more useful paces (even if nobody else cares).

            Of course each day there is a ‘local apparent noon’ when the sun is highest up for the day from the southern horizon wherever you happen to be in the northern hemisphere. But as I indicated, you don’t need your watch at all for this particular task, though it may be useful to roughly estimate when to return (after a leisurely lunch) in time to apply your appointed string length in marking the PM shadow tip point. Then having done so, my geometry teacher would have us swing some equal length longer string arcs both to the north and south of those marked east/west line points to find 2 points on a corresponding perpendicular north/south line.

            Now hereabouts (in southern Calif.) I can assure you that in late June and 6 hours before or after ‘local noon’ (which is for me only 3 minutes ahead of Pacific Standard Time noon — and as you know ‘on average’ due to the sun’s apparent vagrancy throughout the year due to the earth’s slightly elliptical orbit as well as polar tilt) the solar azimuths will be fully 20 degrees north of east or west, as the sun has risen or will set more than an hour earlier/later closer to 30 degrees north of those cardinal directions. As I mentioned earlier, for both your latitude and mine the sun’s rising/setting is due east/west on our horizon only on those equinox dates, which also happens to be 6 hours either side of local apparent solar noon.

            Best regards

          • After playing with the NOAA solar position calculator :
            https://www.esrl.noaa.gov/gmd/grad/solcalc/azel.html
            for awhile, I have to admit that you are right about the sun’s direction 6 hours before or after local noon not being useful for finding due east or west. The sun’s direction in the sky is the most variable at low elevations above the horizon to the east and west.

            It turns out, higher latitudes, and higher sun elevations have less variation over the year with respect to time off day the sun passes a particular azimuth, staying much closer to the equation of time. I had been observing the timing of the sun as it passed points in the sky that were about 25-30 degrees above the southeastern horizon from positions north of 48 degrees.

            I’m even more convinced that using the sun foe surveying is not an easy task A compass is much easier.

            SR

    • If the old survey map shows the compass direction (magnetic North) and shows the boundary (wall),
      then a current measure of the wall against a compass
      will let one calculate how much magnetic North has shifted .

      There is no need for any walls to have been put in on North-South
      or East-West orientations.

    • Not sure why remote rural walls would be oriented magnetically north/south by use of some reasonably sizable instrumental compass anyway,

      Doc Chuck, the answer to your question is, …… remote rural walls were not, are not oriented magnetically north/south, …… unless the property owner or rock wall builder wanted it orientated like said.

      The boundaries of the property being transferred or sold was/is determined by the owner of said property …… and it is said boundaries that are surveyed and their length and orientation to “magnetically north/south” is recorded on the deed or deed plat for said property.

      For example, one (1) boundary line description: “north 40 degrees east 135 feet”.

      All one needs is a “corner” starting point, a compass and a tape measure, ….. no stick, no sunshine, no clock.

  7. Bravo. Early survey lines were run with magnetic compass. In areas NOT governed by the US Public Land Survey System,
    https://nationalmap.gov/small_scale/a_plss.html
    lines would be run based on magnetic north as described in this post. However, PLSS lands were based on “true” north, not magnetic north, even though the surveys were run with magnetic compass. One poster noted that iron ore deposits would make magnetic surveys nearly impossible. PLSS required use of the Burt solar compass to run the meridian lines.
    https://amhistory.si.edu/surveying/type.cfm?typeid=17
    PLSS surveys would not show the magnetic variation in boundaries that non-PLSS surveys would. However, many surveys would note magnetic declination. US Geological Survey and similar maps show magnetic declination as of date of map publication, along with true north, magnetic north, and grid north. Rate of change of declination is sometimes shown.
    https://www.usgs.gov/faqs/what-do-different-north-arrows-a-usgs-map-mean

  8. Several problems with the gross assumptions in this article.

    A) Surveyors used sextants back in the 1700s, not compasses!
    -Remember, surveyors had to determine North,East, West and South with precision plus accurately measure distance rigorously. Compasses, especially during Colonial times were very imprecise.

    B) Walls are movable! I did not see in this eager researcher’s claims, where they determined exactly when a wall was built.
    – Nor did the researcher above account for walls falling apart, getting knocked over, or moved via frost!

    Many thanks to Walter Sobchak February 28, 2019 at 7:11 pm for his excellent post of Robert Frost’s poem about walls!

    One of the greatest American Poems: {Agreed!}
    Mending Wall by Robert Frost (1874–1963)

    SOMETHING there is that doesn’t love a wall,
    That sends the frozen-ground-swell under it,
    And spills the upper boulders in the sun;
    And makes gaps even two can pass abreast.
    The work of hunters is another thing:
    I have come after them and made repair
    Where they have left not one stone on stone,
    But they would have the rabbit out of hiding,
    To please the yelping dogs. The gaps I mean,
    No one has seen them made or heard them made,
    But at spring mending-time we find them there.
    I let my neighbor know beyond the hill;
    And on a day we meet to walk the line
    And set the wall between us once again.
    We keep the wall between us as we go.
    To each the boulders that have fallen to each…”

    Unless the researcher has identified a foolproof extremely accurate method for determining when a wall was originally built, and all moves or repairs, then their ‘discovery’ is misleading.

    • The article mentions that the walls in the surveys were compared to the walls still in existence, and they matched.

    • If a number of walls are parallel, then an assumption that they used a common method to determine direction is possible. However, the method is obviously argumentative!

    • ATheoK, in re point A) – Sextants are used to determine angle between 2 points, usually a star and the horizon. One can be used to determine a horizontal angle to plot a bearing line, but there has to be a previously determined, and marked out, base direction line for a starting point to measure from. Using a sextant doesn’t eliminate the need for some method of determining either true north, or magnetic north.
      If the North Star was used, a surveyor would have to start with a bearing taken at night from the starting corner point.

  9. Dry-stone walls are still alive and well in the hills of Northern England. I wouldn’t be surprised if some of them pre-dated Stone Henge.

    • Michael Hart: according to an economic historian friend of mine many new drystone walls were built in various parts of England, including the north-west, in the early 19th century. Of course many are much older, mediaeval or even prehistoric.

  10. I get the methodology, but that would only give you an angle if the pole was moving perpendicular to these walls (and if you could date each wall). But the NOAA graphic has the magnetic pole moving away from Canada, moving parallel to his observing position. So how could he measure any change in angle…?
    R

  11. Motions in the fluid, molten metals that surround Earth’s core are believed to generate the magnetic field, although the deep layers of the planet cannot be observed directly. The dynamics of the system are not well understood.

    Wow, someone with the common sense and humility to admit that scientists don’t really know what happens in the Earth to create the magnetic field. It’s all hypothesis.

    I loved the honest ” not well understood” instead of the usual “scientists do not yet fully understand”…. like we’ve got 95% of it down to 5 sigma certainty consensus but a few details need filling in.

  12. Pootun’ is trying to annex the magnetic north. Trump needs to draw up more sanctions now !

    Key industries involved in ore extraction and processing as well a key actors in iron, steel and nickel processing must be targeted.

  13. New England is one of the areas where the magnetic declination has been remarcably stable (about 10 degrees west) since the seventeenth centure.

  14. I always knew which way was North, when growing up in my prairie home. Seeing the horizon some miles away and following the track of the Sun each day, gave certainty to the way home, from whatever adventures had beckoned us across the ridges and draws.
    It wasn’t until I grew and left home for the mountains and forests of the East, that I came to understand how people could not know which way was which.

    Still, my youthful curiosity prompted a fascination with compasses and maps and I still have my simple Silva compass purchased more than half a century ago, with money earned by selling redeemable pop bottles, found in roadside ditches.

    The local declination back then, was 11 degrees+E. A decade and a half ago, it had moved to 4+ degrees, the same as that of Baghdad. It now stands at 3 degrees 35 minutes+E.
    Nothing in this world remains unchanged.

  15. Analysis of the movement of the Earth’s magnetic poles over the last 105 years demonstrates strong correlations between the position of the north magnetic, and geomagnetic poles, and both northern hemisphere and global temperatures. Although these correlations are surprising, a statistical analysis shows there is a less than one percent chance they are random, but it is not clear how movements of the poles affect climate. Links between changes in the Earth’s magnetic field and climate change, have been proposed previously although the exact mechanism is disputed.
    https://adriankerton.wordpress.com/climate-change-and-the-earths-magnetic-poles-a-possible-connection/

  16. Jonkers, A. R. T. 2003. Earth’s Magnetism in the Age of Sail. Johns Hopkins University Press. A fascinating account about the attempts to navigate, especially with longitude, based on log books. Besides horizontal variation there were also measurements of vertical dip. Very frustrating.

    Also this paper–Jackson, A., Jonkers, A. R. T. and M. Walker. 2000. Four centuries of Geomagnetic Secular Variation from Historical Records. Philosophical Transactions Royal Society London. A. 256(1768):957-990.

  17. On my wanderings around New England’s forests back in my trail running days in Massachusetts, New Hampshire, and Vermont, these old stone walls that used to define farm land and planting fields are everywhere.Those that still exist (not destroyed by modern development) are mostly buried in forest now. And I mean they are everywhere. Farming this rocky poor soil was certainly difficult.

    The colonist-farmers had to clear large hard wood tree forests with briar thickets in any open space that could get sunlight. They had to move many 100 to 500lb single rocks with ox and mule and then stack them. All those rocks were left over from the glacial retreat and dumped on the ice-scoured ground, probably 10,000 years earlier. Since that retreat, the soil creation processes of nature slowly covered them to be just below the visible surface. Fine for forests and grass field — horrible for planting annual crops. But clear them they did, and they used those rocks to make their walls, a defining “I own this land within these walls” statement as a landowner.

    As an aside, those low rock walls were great ambush positions for hit and run colonial militia attacks against hapless British soldiers trained to shoot from defined formations in open fields facing a similarly aligned enemy.

    These were also the colonial farms that fed Boston, New York, Philadelphia. But once the Oregon Trail opened up and word spread of rich, deep fertile lands free for the taking (don’t mind those pesky natives though), that farms began to be abandoned wholesale.

    When you walk around so many of those forest enclosed rock walls, you close your eyes can imagine the 300 years earlier of men and boys toiling everyday, sunrise to sunset to dig up, move and build those walls sunrise to sunset. Those were hardy people. Today, we cannot even imagine what a hard life that was.

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