[Aztlan] Red soils in the Yukatan - a short history

[Bruce Rogers]

Listeros,

In response to several comments about the reddish 
ocher soils in the Yukatán, I submit these 
hopefully simplified notes.  First, however, I 
want to point out that little work has been done 
in the Mayab as far as general geology, karst 
studies, and especially soils, thus the following 
comments are somewhat preliminary in nature.

Dave Hickson mentions the color of the residual 
soils - kancab luum - is striking.  The general 
color ranges from yellowish red (Munsell 5YR 5/7) 
to a rich reddish dark brown (Munsell 7.5YR 4/3). 
This coloration is typical of soils that have 
developed over a long period of time.  The karst 
soils of Yukatán often take upwards of several 
thousands to tens of thousands of years to form. 
These soils represent the final, heavily leached 
materials left after nearly everything else 
soluble has been carried away or absorbed into 
the covering vegetation.  The ocherous colors may 
also be due in large part to fluvic and humic 
organic acids in the soils themselves, a fact 
unappreciated by many.

The soils of the Yukatán are residues from the 
weathering of the limestone itself.  Because the 
limestone is so pure the surface of the peninsula 
is slowly disappearing at the rate of about 1 mm 
per year.  Several percent of impurities were 
originally included when the limestone was laid 
down over the last 100 million years or so as 
sandy, silty, or clayey layers.  These impurities 
include such materials as pyrite, gypsum, 
aluminum and iron oxides of several flavors, and 
other very minor materials.

These insoluble minor impurities will add to the 
meager soil present on top of the limestone.  The 
pyrite will readily weather to form a weak 
sulfuric acid that will accelerate the weathering 
of the limestone and leave a residue of iron 
oxides.  These iron oxides, in turn, are at least 
partly dehydrated to form hematite; thus the 
yellowish to reddish cast of many tropical karst 
soils.  The few aluminum compounds present are 
rather stable and remain as orangish-yellow 
deposits of bauxite, a mixture of several 
aluminum-rich minerals that can, if concentrated 
enough, be mined as aluminum ore.

The gypsum is very soluble in water and will 
"gleet down the crannies" as the British are fond 
of saying, leaving more residues of originally 
incorporated iron oxides and such.  Silica 
compounds - sand - are remarkably soluble in 
percolating water and often is totally absent 
from karst soils.  The balance of the soil is 
made of residual clay minerals; generally these 
are buff-colored clays.  These include 
attapulgite/palygorskite clay that is deposited 
in special conditions present in saline playas 
and deserts.  In the middle of the limestone 
package that comprises the Yukatán are several 
units of gypsum and "red beds" of iron oxide-rich 
sediments that indicate that sometime in the past 
the area was above sea level, arid, and such 
deposits as attapulgite were precipitated.

(A digression here: attapulgite is another name 
for the mineral palygorskite.  The name 
attapulgite comes from the large deposits in 
Attapulgus, Georgia; the name palygorskite come 
from the Popovka River area in the Ural Mountains 
of Russia.  The crystal structure of attapulgite 
is that of felted masses of microscopic tubes. 
When used to make Maya blue, the indigo is 
tightly bound inside the tubular crystal 
structure, hence its remarkable persistence.)

It has known that the current atmospheric 
conditions allow large amounts of dust to be blow 
from the Sahara region across the Atlantic Ocean 
to settle in the Americas.  Much of this 
microscopic dust is iron oxides or iron 
oxide-colored minerals.  It is reasonable to 
assume similar conditions have been present 
during the last 10,000 years of the Holocene - 
the time after the end of the Pleistocene Ice 
Ages. It is possible that pulses of clay-sized 
material were lofted into the atmosphere during 
especially arid times in the Pleistocene and 
Holocene, but it is unlikely that the reddish 
soil color is due to one massive windstorm in the 
past.  While this airborne material does add an 
amount of clay-sized material to the Americas 
soils over time, it does not comprise the bulk of 
the terra rosa soils present in the Mayab.  The 
main point here is that this process has been 
going on for some time and that the bulk of the 
soils are made of insoluble residues of the 
limestone itself.

John Hoopes commented on the ocher-colored soils 
in Actun Loltún.  The soils in Actun (Grutas) 
Loltún are soils derived from the overlying 
surface (Actun Loltún glosses to stone covered 
[=cave] + stone flowers [=stalactites].  The 
large entrances of this 1.5 km-long cave admit 
large amounts of seasonal rainwater as do the 
multitude of skylight entrances and narrow 
bedrock fissures.  This washes considerable 
amounts of clay and silt into the cave passages. 
The area near the middle of this cave at 
Stalactite Junction and the Paintings Room 
contains fairly thick deposits of reddish clay 
and silt, but I personally would not call much of 
this material a "red" ocher deposit since much of 
it is colored the strong brown (Munsell 7.5YR 
5/6) and yellowish red (Munsell 5YR5/8) of 
goethite or "limonite."  In other parts of the 
cave small patches of brownish yellow colored 
soils (Munsell 10YR 7/8) apparently have been 
derived from naturally decrepitating limestone 
from the cave walls mixing with flaking secondary 
crusts of gypsum.  In other remote parts of the 
cave, relatively large amounts of organic 
materials have mixed with the residual terra rosa 
to form dark brown soils (Munsell 10YR 4/6); in 
the 8,800 year old "mammoth BBQ" area of the 
Huechil Passage are such soils.

There is evidence suggesting the Maya mined some 
of this clay in Loltún for their own uses - 
perhaps as pottery clay.  Certainly other cave 
sites have abundant evidence for such activity. 
Near Chichen Itza, several km of the extensive, 
complex, and confusing network of passages in 
Actun Kaua show where nearly a meter of clay has 
been removed for some purpose.  In areas of Actun 
(Grutas) Balancanche there is evidence of soil 
removal as well as mining of speleothems 
(stalactites, stalagmites, etc.) by the Maya. 
Most probably this was done to secure suitable 
materials for making pottery.  The clay would be 
the bulk of the material used and the calcite 
speleothems used as a tempering agent.

The rock art in Actun Loltún includes profiles of 
human heads, depictions of pottery, abstract 
designs , and negative hand outlines of at least 
two different kinds.  Most of the rock art in 
Loltún has been rendered in black, most probably 
finely ground charcoal or possibly one of the 
multitudes of manganese oxides.  The charcoal 
came from burning surface vegetation, of course; 
possibly copal.  The manganese, however, may have 
come from deposits in the cave or other seasonal 
streams and pools during the waning stages of the 
Ice Ages prior to 10,000 years ago.  Where there 
is water there may be manganese-precipitating 
bacteria that would have formed lusterous black 
and blue-black deposits. To my limited knowledge, 
no one has done any detailed investigations of 
the black pigments in Loltún.

There is a small corpus of bright red rock art in 
Actun Loltún, however.  Near the back of the 
large Inscription Room and near the Ahau Entrance 
are nearly 0.3 m-high, painted stick figures. 
The red (Munsell 10R 4/8) mineral pigment used in 
these figures is most definitely from some other 
source since it is so different in color from the 
soils in the cave.

Now you know more than you ever wanted to know 
about terra rosa, Maya soils mining, and mammoth 
BBQ sites.

Cheers,
Bruce Rogers, earth scientist on a good day