Along Interstate 85 in Virginia starting at about Mile post 38
Northbound and ending near Milepost 52 Northbound my magnetic compass
just jumps randomly and constantly. The other day I took an old Boy
Scout compass on a trip to Richmond and it did exactly the same thing
in the same area. I can't identify what the cause would be. Aero
flight maps do not show any compass anomalies for that area. Yes, Fort
Pickett (now closed I think) is northwest of this location maybe 7
miles. It's the same effect, same area, north or south bound. If I
ever find a State Trooper in a position where I can ask him the
question, I will. Oh yes, at least on the surface it is no an obvious rocky area.

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Clarification of Question by harttx-ga on 25 Apr 2005 12:16 PDT

Some interesting research today: Returning from Richmond, VA I noted
the beginning og the "anomaly area", which I make as about 12 miles
long. So I drove about 5 miles and got off at an exit (DeWitt, VA I
think.) As soon as I hit the ramp the compass became steady as a rock!
To be sure I drove east and west about a mile each way - no jump, no
swing. I got back on the 85 and off it went again. I drove down to the
McKenna, VA exit and did the same off road survey with the same steady
results. And returning to the 85 heading south and trying to ID as
closely as I could the spot where the "anomaly" ends, I noted a
violent jumping just before and during the crossing of what I believe
to be the Nottoway River Bridge. Also talked with some State Troopers
yesterday - didn't do too well, one said it's out of their area, the
other said it's planes from Fort Pickett, and compasses don't work in
cars anyway! Huh what? Anyway now the road itself is suspect as in
something buried beside it or beneath it, maybe fiber optic or
whatever, I have no clue. The observable construction of the road does
not change in the effect area. And the beginning and end of the effect
are not recognizible as anything more than another spot in the woods,
or next to the river.

Hello harttx,

I have posted this as a comment because I don't have a cast iron (pun
intended, see my comments ;) answer for you, but I think it's probably
linked to the fact that Iron ore mining and processing has been going
on in the area since the 1620's.

See:

(Chesterfield County)
"A number of high-amplitude magnetic anomalies appear in the magnetic
field gradient data that are thought to be associated with iron
production, including extensive slag deposits and the possible
location of the blast furnace. Ground penetrating radar data shows an
anomalous reflection in the vicinity of the suspected blast furnace,
which is partially beneath the modern roadway."
http://www.archaeophysics.com/falling/index-main.htm

"Chesterfield's Location
Location in Virginia"
http://www.co.chesterfield.va.us/Maps/chesmap2.asp

Also look at the number of Iron works currently in the area:
://www.google.com/maps?li=lmr&hl=en&q=iron+loc:+Blackstone,+VA+23824&num=10&hl=en&ie=UTF-8

If you think this answers your question, just let me know.  Or it
might be that another researcher can answer your question with more
accuracy.

Regards

THX1138

My problem with the otherwise very logical answer of thx1138-ga is
that the area he is referring to is quite a bit north of the area I
refer to by I'm guessing 40 to 60 miles(starts near Dewitt, VA) AND on
the same trips (which is all) where I get the anomaly from Mile 38 to
52 of the 85, I also usually pass through a large area of Chesterfield
County without so much as a bounce of the compass. Maybe we've found
"Cheney's Cave" (g).

Hello harttx,

No, "Cheney's Cave" is a good deal North of the area you are concerned
with. Have a look at the maps on this site ;-)
http://www.cryptome.org/site-r/site-r.htm

Regards

THX1138

Does the same area affect operations of two way radios. I experienced
a situation where two way radios simply would not function between two
towns only nine miles apart. The reason was a massive underground ore
deposit that simply soaked up the radio waves.

Pugwashjw --

The radio waves might have been absorbed by many things, even leaves
and pine needles.  A few years ago a friend was in charge of
installing 825 MHz radios for Metricom, a cell-based communications
company.  Installations had been going fine until they installed a
group on the Hewlett-Packard campus.  Radios would send and were
operating properly but other radios couldn't receive the signal.  It
turned out that the pine trees on the H-P campus were absorbing the
radio energy and that cell stations had to be moved.

My apologies to Harttx for taking this off-topic.

Best regards,

Omnivorous-GA

Hi Omni. Your info is very interesting. Were the effects through
needles on the trees or a buildup on the ground. And April 1 is long
past. with respects. Pug.

I suspect that your anomaly is caused by one of the following:

1. A mineral deposit. As far as I know, the iron deposits in your are
are "bog iron," which seems unlikely to create a major magnetic
anomaly. To pursue this, I would begin by looking at bedrock geology
maps of the area, looking for iron-bearing formations, igneous
intrusions, etc. Anything that would cause a compass needle to deviate
like you describe would probably show uf as an aeromagnetic anomaly,
and you said you have checked this.

2. A large man-made concentration of iron or steel. You mentioned that
it jumps around on a certain bridge. A steel bridge? You might try
walking around with your compass near cars, steel structures, etc. to
see what effect they have. I happen to be one of the people who
investigated the Falling Creek Ironworks that someone mentioned, and I
think you would have to be right on top of something like that to see
an effect on your compass.

3. Electromagnetic fields. Are there any high-tension power lines in
the area in question? There may be something under or beside the road
as well. You mention the compass needle "jumping." have you determined
whether this occurs with movement, i.e. moving through static
variations in magnetic field, or does it jump when you are stationary,
which would indicate some sort or EM signal. I have seen pulsed
magnetic anomalies that I suspect were caused by military activities,
but I don't imagine such things being strong enough to effect your
compass. Cheney's cave? Perhaps.

4. Your car. The trooper was right about compasses in cars. They can
point north, but I wouldn't rely on it. they can point different
directions in different parts of the car. Try getting out of your car
and walking around to get a better idea of the size and geometry of
the anomaly without the movement and magnetic fields associated with
your vehicle.

For lack of a definitive answer I asked the VDOT Engineer in Residence
for that area. Here is his response:

"Don, it could be that  the reinforcing concrete in the concrete pavement
changes orientation in this stretch of road.  As you may know, there were
different types of concrete pavement constructed over the years.  That
stretch may have an unusual arrangement of the structural steel - and then
again, it may be something entirely different.  Good luck in your search."

Ray Varney, PE, LS
Petersburg Resident Engineer
804-863-4004

Now I have since checked his response against the actual pavement used
and I would declare he has the best answer. At both ends of the
anomaly area there is a precise change from open concrete paving to
what I assume is concrete with asphalt overlay paving. I would
personally send him the $5 except he's a Public Employee and cannot
accept monetary benefits from the private use of his position. (g)

That is interesting, and thanks for posting.  At least I appreciate it.  :)