McCoy Mountains

Limonite Cubes

Text by Delmer G. Ross - Photographs by Karen A. Ross

“Limonite” is a term used to describe any iron hydroxide that has not been more exactly identified through the use of elaborate chemical analysis.  Obviously, then, there are various different kinds of limonite.  The one with which we will deal here is called “pseudomorphous limonite,” after pyrite.

By Michael Dorausch from Venice, USA - McCoy Mountians, CC BY-SA 2.0,

Pyrite, also known as fool’s gold because it has tricked many into believing it was the real thing, is iron sulfide, an iron ore.  Under the proper conditions, though, pyrite can become iron hydroxide, or limonite.  Its external appearance remains essentially the same, but the composition has been altered.  Pyrite cubes have become limonite cubes, which have also been called “Indian Money” because people believed that the strangely shaped rocks could only have been shaped by man’s hand, not by nature. 

On the road to the collecting site

The McCoy Mountains, also known as the Ironwood Mountains, situated a few miles northwest of Blythe, California, can be a good source of limonite cubes.  Small quantities have been found from one end of the range to the other.  Probably the best area to search for them is on the low, desert-pavement-covered alluvial fans known as “bajadas,” a Spanish word appropriately meaning “down slope.”  The bajadas are located along the southeastern flank of the mountains.  Rain runoff has washed the cubes from their original matrix – usually either soft mica schist or much harder quartz – down to the flatter areas below.  Of course, that same runoff water transported much other material to the same locality. 

The resulting bajada was further eroded by water and wind to the point that much of the fine material was carried away, leaving a layer of closely spaced rocks on the surface of the bajada, a surface that resists further significant erosion unless something happens to disturb it.  Such a surface is known as “desert pavement.”

Desert Pavement

If one looks carefully, limonite cubes generally can be found in the desert pavement on the bajadas at the southeastern extreme of the McCoys, at the eastern edge of California’s Colorado Desert.  Collecting a cube, or removing any other rock, disturbs the surface sufficiently to speed up the erosion process until the surrounding area descends enough for another rock to replace the one taken, thus restoring the integrity of the protective desert pavement.  This is why an area that has been searched until there literally are no more cubes will suddenly “sprout” new ones after a good rain.

Limonite cubes

To collect limonite cubes in the McCoy Mountain area, drive west from Blythe, California, on Interstate Highway 10, for approximately six miles to the Mesa Verde exit.  Get off the freeway there, cross Mesa Drive, then continue westward on Black Rock Road, the frontage road that parallels the interstate highway.  For those using GPS readers, the junction of Mesa Drive and Black Rock Road is located on the north side of the interstate highway, at 33°36.659' N by 114°43.640' W.  Drive 1.3 miles west on Black Rock Road to 33°36.573' N by 114°44.981' W.  At that point turn north on a narrow road that is almost hidden by tall creosote bushes.  The first mile of this road can be very sandy.  Much depends on how the last rainstorm may have left it.  Therefore, unless your vehicle has four-wheel drive, you should probably get out and check the road before driving over any portion of it.

Assuming you decide to continue, about 1.4 miles north of Black Rock Road, at 33°37.716' N by 114°45.161' W, the road will climb steeply to an area of rolling desert pavement composed largely of strangely rounded pebbles.  This is the northwestern extreme of Pebble Terrace, where the Colorado River once deposited many square miles of such worn rocks.  Although in some places it has been washed away and in others covered by wind-blown sand, this pebbly formation extends some 12 miles south to a point west of the community of Palo Verde.  Because of its nature, Pebble Terrace contains rocks from nearly the entire drainage area of the Colorado River above this point. 

Rockhounds have discovered the rounded pebbles can be almost anything, from various kinds of agate to petrified wood.  Because almost all of the surface pebbles are covered with desert varnish, the trueThe rounded pebbles can be almost anything, from various kinds of agate to petrified wood. identity of some can be well hidden.  Knocking off a small chip with your rock hammer, though, should take care of that.  Pebble Terrace is a collecting area that can be enjoyed by all, from the youngest rock pup to fully grown and experienced rock hounds.  Rockhounds should be warned, though, that much of Pebble Terrace to the north of Interstate 10 is on private property or on gravel mining claims.  To avoid problems, any extensive collecting doubtlessly should be done where it is more welcome.  The area to the west of Palo Verde is probably the best known and most easily accessed.

To reach the limonite-cube collecting area, though, continue driving northward, past the entrance to private property, to a wide wash 3.7 miles from Black Rock Road.  Cross the wash, taking care to cross it rather than to drive up it.  In another 0.1 mile, at 33°38.316' N by 114°45.912' W, turn west and drive 1.1 miles over a meandering road along the north bank of the wide wash you just crossed.   About 0.35 miles from where you turned onto it, the road climbs slightly onto a desert-pavement-covered bajada, but it continues along the north side of the wash.  At 33°40.105' N by 114°46.901' W, where to your left in the wash you will see the rusted remains of an old delivery truck, you will reach a crossroad that descends into and then crosses the wash.  Turn left and drive across the wash 0.1 mile to 33°40.033' N by 114°46.913' W, on top of the bajada on the other side.

The collecting area begins at 33°40.033' N and 114°46.913' W, at the eastern extreme of a long bajada, and continues 0.9 mile west, up the bajada, to about 33°40.053' N by 114°47.720' W.  Look for shiny flat surfaces that, despite being jet black, reflect brighter than the equally black surrounding desert pavement.  The cubes are especially noticeable in morning sunlight, from about eight to eleven o’clock.  If you are unsuccessful here, try the bajadas to the north or south of this one.

Fuel and limited services and supplies can be obtained at the Mesa Verde exit from Interstate Highway 10.  Accommodations and additional services and supplies are available in Blythe, about 10 miles east.

Delmer G. Ross is a Professor of History, La Sierra University


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