Strange Places: 5 Clam Shells, Cacti and Limestone
With this column, we leave Africa and return to the New World, specifically some of the odder places in the Chihuahuan Desert of northeast Mexico and the adjacent U. S. A. The Chihuahuan Desert is huge, over 250,000 square miles—more if you include its eastern and southern borderlands, home of many of its most interesting plants. Much of it resembles any desert landscape, gently sloping plains, low hills, eroded, sharply angular transverse mountain ranges, but its most intriguing, almost eerie habitat covers only relatively small portions of its vast expanse. These are its limestone outcrops, often composed to a considerable degree of masses of large, fossilized Cretaceous era clams. These habitats provide a home to a number of interesting cacti as well, some widespread, some fairly localized, and many of them very white in color.
We’re fortunate, because some of these odd limestone deposits sit right in Big Bend National Park, on the Texas side of the U. S. /Mexico border, and accordingly is more easily accessible for people here—you don’t even need four wheel drive to reach it! The main limestone formation in the area is known as the Boquillas limestone, and it’s associated with huge numbers of fossilized clam-shells, from the genus Inoceramus, large clams that occasionally reached several feet in length, but which more typically were six or eight inches long. At various locations the distinctive, swirlingly ridged shells actually form a good portion of the rock itself, and are inextricably mixed in with their matrix, making the place a poor site for fossil collecting (even if it were legal there, which it certainly isn’t). More interesting from our point of view, however, are the several cacti species that grow here, and the overall unearthly aspect of these limestone areas. I’m also going to take the opportunity to mention some of the other more unusual cactus inhabitants of the Big Bend region that don’t grow on the limestone, but that deserve a bit of discussion, as this protrusion of the heart of the Chihuahuan Desert into the United States includes many of our country’s most interesting cacti and succulents.
Boquillas limestone is strongly bedded—that is to say it was deposited in big, quite flat layers, and this sedimentary bedding, whether still level as originally laid down, or bent and folded over the eons, dominates much of the Big Bend landscape. Typically it manifests itself in the form of fairly small, more-or-less level areas that break up, usually on their south side, into a series of terraces, sort of stair-stepping down through the layers. The northern and central sections of these small plains have undergone a considerable degree of erosion and weathering, so the ground usually consists of white, flattish pebbles overlaying a limey soil and interrupted by small, rocky outcrops. The vegetation here is typical of the Chihuahuan Desert: big clumps of Agave lechuguilla, along with the “false agave,” Hechtia scariosa (a terrestrial bromeliad), rhizhomatous, leathery stemmed Jatropha spathulata, patches of the gray, reed-like Euphorbia antisyphilitica, occasional ocotillos (Fouquieria splendens), along with creosote bushes and other non-succulent xerophytic vegetation Cacti include several species of Opuntia, both pad types and chollas of various configurations, a few species of Coryphantha including the clumping, dark-green stemmed C. macromeris, and members of the closely related genus Escobaria such as E. dasyacantha and E. tuberculosa (the taxonomy of these plants is complex—some botanists think Escobaria is simply part of Coryphantha, leading to an entirely different set of species names). The most visible cactus on these flats, however, consists of large populations of the very widespread Echinocereus pectinatus. Most of the other cacti and succulents I’ve mentioned so far don’t particularly vary whether they’re growing in limestone or not, but the echinocerei in these parts, aside from being taller than usual (some close to two feet in height), also are very white, sometimes banded with pink or red and black. Their dense, pectinate (arranged like the teeth of a comb, without protruding central spines) spination protects them from sunburn, and presumably provides a certain degree of armor against predation, but from a purely aesthetic point of view, the limestone Echinocereus of Big Bend must be among the most attractive forms of this very wide-ranging species. Several other Echinocereus species live in Big Bend as well, widespread forms and rarities such as E. chisoensis, with bristly spine clusters and somewhat spiraling ribs. This species, with four and a half inch magenta flowers growing from smallish clusters of seven or eight inch stems, tends to grow in sandy flats rather than on limestone shelves.
Several other of the rarer cactus species on Big Bend also avoid the limestone areas, among them Mammillaria pottsii, which grows on granite. With its distinctive spination and small, bell-shaped red flowers it’s unlike any other United States mammillaria—it is rare in Big Bend, but quite common farther south in Coahuila State in Mexico. Epithelantha macromeris is another small cactus, round as a button and completely covered with pure white spines that look almost like close-cropped fur. It grows as far north as Carlsbad Caverns, in New Mexico, at quite high altitudes, and in Big Bend also it’s found at higher elevations, growing on buff and orange colored granite and so completely negating its camouflage. Echinomastus warnockii is yet another miniature Big Bend barrel, quite rare and limited in its range, but not a limestone inhabitant. Rocky areas, whether limestone or not, provide a home to the rare, localized, and attractive Escobaria albicolumnaria, attractive upright clusters of small, densely spined columns with pretty pink flowers. Yet another highly localized Escobaria species from Big Bend, E. duncanii grows in crevices on flat-topped small plateaus. The plants are very small, usually solitary, with tuberous roots and delicate, almost feathery-looking masses of white spines. They look like small piles of cobwebs from a distance.
Those portions of the limestone flats, where the soil becomes more rocky, made of limestone plates that adhere together closely with little actual soil between them, do support populations of interesting and unique plants. Among these, Echinomastus mariposensis is a small, solitary globular cactus, no more than a few inches tall and completely covered with glistening, glassy white spines. Although it also grows sporadically outside the park boundaries and just south into Mexico, this pretty little plant has an extremely limited range. Coryphantha echinus, a small, quite spherical plant with long central spines, pure white and tipped with black, grows both on these flats and in the nearby rocks, sometimes as a solitary sphere, sometimes in quite large clumps. Another white limestone specialist, Epithelantha bokei, rare in the United State (confined to the southern Big Bend region in fact), but more widely scattered about Coahuila in Mexico, is even more rounded and more densely covered with tiny, harmless white spines than E. macromeris. It may be no more than coincidence, but the plants distinctly resemble deer droppings, possibly making them quite undesirable in the eyes of creatures which might otherwise want to make a small cactus snack out of them. Divorced from this image, however, the plants are really extremely attractive, among the most interesting and highly specialized cacti in the United State. E. bokei may grow in the central, rocky parts of the limestone flats, or may grow near the even more rocky, less decomposed limestone near the southern terrace edges.
The very edges of these limestone terraces are the home of Ariocarpus fissuratus, the only member of its genus that crosses the Rio Grande into the United States. Since these terrace edge drop-offs face south, they receive maximum sunlight, because they end in mid-air, they also experience the maximum drainage possible. Epithelantha bokei is one of the few cactus species other than the ariocarpus that can survive this harsh situation, but in these locations it hunkers down between bits of broken rock which provide a bit of shelter. The ariocarpus, on the other, survive completely out in the open, sometimes even managing to insert their way into the vertical faces between the terrace levels, and subsist on a soil that consists of absolutely nothing but plates of limestone. They live on their own decaying tubercles, as I’ve mentioned in other columns, surely one of the more amazing examples of nutritional economy in nature.
These species make up most of the more interesting cacti of Big Bend, but their habitat, alien as it may seem, is less striking than the little flat-topped ridges of the nearby limestone hills. These hills, made largely of uplifted plates of Boquillas limestone, sometimes support a population of Ariocarpus fissuratus and little else on their hump-backed summits. Their dissected slopes fan out in a series of small ridges and gullies, restricted to tones of white and gray. Many of these lateral ridge tops protrude forward horizontally, making them very easy to walk on. The dominant cacti here (and there’s little else in the way of vegetation) are medium-sized barrels of Echinocactus horizonthalonius, arranged with thick spined ribs and quite gray in color. Though uncommon in the United States, this is a very widespread plant in Mexico, growing throughout the Chihuahuan Desert. On the ridge tops, the plants arrange themselves in regular intervals, and as one walks along, the geometrically plated limestone rings musically underfoot, almost like a landscape made of flattened bells. The only relief from the white and gray comes from bands of Selaginella lepidophylla, the “Resurrection fern,” a fern ally that expands its flattened yellow-green fronds during the rainy season, and dries up into a tight, almost black ball during the dry part of the year. These selaginellas grow along the more moisture-retaining parts of the arching terraces, and are so abundant as to make entire hills seem zebra-striped, alternating between bands of white limestone and black dormant plants. The selaginellas, here, and far to the south as well, serve as repositories of moisture and wind-blown bits of soil, and so become nurseries for many cacti that as seedling can’t withstand the droughts they easily endure as mature plants.
Walking these limestone flats, terraces and hills seems like walking on a moonscape, that is if the moon were made of white limestone, humped up into ridges marked with fossil clam shell remnants looking like the swirling ridges of giant petrified fingerprints. Although Big Bend National Park is remote, it’s one of the few places in the United States where this exotic landscape exists, along with its population of bizarre cacti, and seeing these landscapes and vegetation in the context of its surroundings more than justifies the occasional lengthy trip.
|
