Okay, so I get 2 emails this morning asking "what are you talking about?" in reguard to my post below about "subtile gradation change."
I'll just post my answer [explanation] right here for whatever it's worth.
I have a number of pictures that illustrate the scenario I described, BUT even high-def digital prints are all but useless. These SUBTILE variations are [sometimes] hard to idenity even when you're standing there under perfect conditions.
Basically, the scenario is something like this:
The natural ground consists of PRIMARILY sandy soil with a random scattering of 5 to 25mm diameter pebbles, with nothing bigger than a golf ball within say 50 yards.
When the original surveyor SETS the Corner [post], he digs a hole about 12 inches in dia. and 24ins. deep, and sticks the post in it. He then shovels the dirt back in the hole, tamps it down, and then "raises" a MOUND of dirt (usually described as "earth and stone") about 12-18ins. high and 2-3 feet in dia. around the post. Pretty standard for much of Utah and Nevada.
As the years go by the termites and wood ants get after the post, the wind and rain erodes the "fine" sand and soil (say the -50/100 stuff), and after a hundred years or so, there is no topographical evidence of the mound what so ever. Whatever remains of the post is BURIED, (or scattered around as toothpick sized remnants usually hauled away by birds or pack-rats to build nests years ago), and all that "shows" on the surface is an ANOMALOUS concentration (usually 2-3 feet in diameter) of the "larger" pebbles that were less susceptible to erosion by wind and rain.
Ya see...the act or "raising" a mound back in say 1904 (or whenever) had the net effect of CONCENTRATING these larger pebbles AROUND the post. This also [usually] created an area around the mound (say about 6 feet or so in dia.) that is a little "lower" than the surrounding natural ground (where the original surveyor "scooped" up the surface "dirt" to build the mound). This "depression" often "fills in" with the FINE grained sand and dirt washed (or blown) out of the mound, which can further accentuate the "anomoly" in the middle.
After you have seen a couple of these, they tend to STAND OUT quite readily. BUT, it's been my experience that you can take all the pictures you want, BUT generally speaking, even I can't REALLY "see it" very well (if at all) in the photos.
Of course AFTER you have probed the area and found the "soft-spot," then excavated for the rotted wood (or bug-dust), the physical evidence that first prompted you to investigate further is long gone.
I remember one particular Mineral Survey Corner that was right alongside a jeep trail out in Goldfield. The [above described] gradation pattern was PERFECT, one of best I have seen. But probing, digging, and swearing failed to produce ANY organic evidence what so ever. I had of course "shot" (and photographed) the center of the "mound" (anomoly) BEFORE screwing with it, and I elected to ACCEPT the gradation pattern as the "best available evidence" of the original Corner's position (it fit within about .2 feet in both directions). If I ever have to defend THAT decision in court, the photo's ain't a gonna help much.
Fact or opinion?
BOTH! (as far as I'm concerned).