As the Wild River Runs
At 3,000 meters (10,000 feet) the golden eagle glides on outspread wings, his head cocked down so he can watch for signs of life upon that motionless desert rolled out like a relief map below him. Largest of Big Bend’s airborne predators, the eagle needs an enormous hunting range, and riding the warm air currents high above the border, he can see it all: the flattopped and arched mountains, the sun-bleached lowland, and the silver Rio Grande disappearing and reappearing as it runs downstairs through steep canyons and open valleys. Tilting his two-meter (7-foot) wings, the eagle slipslides for a closer look into a canyon, spots the wake of a surface-swimming snake, folds his wings, and dives like a fighter jet. Before the snake even senses its peril, it is snatched aloft and hangs wriggling in the eagle’s talons as the great bird, feathered to the toes, lifts and flies up the canyon with mighty, measured wingbeats.
The waters dripping from the hapless snake’s body come from mountains far to the south and north. The Rio Grande begins in springs and snows high in Colorado’s Rockies, but backed into reservoirs and doled out to irrigate New Mexico and Texas farmlands, it may hardly even flow below El Paso. What gives the river a new lease on life is the Rio Conchos. This beautiful stream rises in the western Sierra Madres and flows northeastward across Mexico, cutting canyons of its own and joining the Rio Grande at Presidio, 160 river-kilometers (100 river-miles) above the park. Some geologists say it was the Rio Conchos, and not the Rio Grande, that cut those gorgeous canyons in the park. No one knows for sure. But once the river trapped itself, all it could do was dig deeper and deeper by processes that are still at work today.
A walk along a sandbar will show you that the river functions as a practical sorting machine. The water rolling by is so laden with sediment that you cannot even see rocks 13 centimeters (5 inches) below the surface. On the bar itself a layer of curling and flaking mud lies on top of the larger stones and gravel, which have fine sand deposited between them. The heaviest rocks settle out first, then the sand, and finally the finest particles. Water is a powerful lifting and pushing tool, but these water-borne abrasives do much of the river’s work, wearing out the rock, undercutting cliffs, deepening and widening the canyons. It goes on at normal stages of water where the river runs less than a meter (2-3 feet) deep, during floods when it crests at more than 6 meters (20 feet), and even during droughts when in many places the river is too shallow to float a boat. So in the slow course of geologic time the mountains are worn away, spread across the valleys, and carried out to sea.
The only streams that have a chance of leaving the desert alive are those whose water sources lie outside the desert. There are few such rivers in the world: the Nile, the Indus, the Tigris and Euphrates, the Colorado, and the Rio Grande. And what a wealth of water-loving life the wild river brings to the Big Bend desert. You can hang big catfish by the gills from your saddlehorn and have your horse walk off with fishtail dragging the ground. So the fishermen tell you.
Cotton and food crops grew during the first half of this century at Castolon (top) and Rio Grande Village. Both floodplain settlements are popular stops for park travelers today.
A third riparian settlement, Hot Springs, offered resort accommodations in the 1940s.
Life in this watery world is sustained by a food pyramid based on a super-abundant supply of tiny bottom organisms. A third-meter (1-foot) square of riffle bottom has been found to contain more than 100 organisms. Most are larvae of flying insects: stoneflies, mayflies, dragonflies, damselflies, water and terrestrial bugs, various kinds of flies, midges, and dobsonflies. These curious little creatures have evolved ingenious ways of living, breathing, and eating underwater. Some worm-like caddisfly nymphs build protective cases around themselves, gluing pebbles, bits of shells, and plants together with saliva. They have three pairs of legs up front sticking out of the case and a pair of hooks holding on to it behind, so they can drag their houses with them as they feed. Damselfly larvae breathe through three leaflike gills that project from the hind end of the abdomen, and when warm weather comes they crawl ashore, split their skins, and emerge as gossamer-winged adults. The gills on stonefly larvae extend from the head and thorax, while mayfly nymphs have seven pairs of gills standing out like feathers along the sides of the abdomen. When oxygen is in short supply, mayfly larvae vibrate their gills rapidly so as to quicken the flow of water along their bodies. Some aquatic larvae build nets to catch dinner; a caddisfly nymph may spin a kind of silken windsock that he hangs underwater with the narrow end downstream, using the pressure of the current to keep his prey trapped. Some aquatic larvae eat microscopic plants, some eat insects, and some eat each other. Large dragonfly nymphs may even catch and eat small fish. Larvae are consumed by fishes, frogs, and turtles.
A rafter hauls out on a sandbank inside Mariscal Canyon, the middle—and most sheer-walled—of Big Bend’s big three canyons. River runners thrill to Tight Squeeze, in Mariscal Canyon, where a rock slab as big as a car compresses the river into a tricky gap.
Probably the best way to get to know the river is to get out on it. An easy run is through Hot Springs Canyon, by 90-meter (300-foot) cliffs and over nice little rapids. You put in at the site of the old Hot Springs spa, and take out at Rio Grande Village, having to paddle only at riffles.
Suppose it’s early on a fine October morning and you’re floating along with the current, watching the sky, clouds, cliffs, and river cane reflecting blue, white, tan, and green on the glossy brown surface of the water. The river is too muddy for you to see what lives in it, but you can see the signs: a spreading circle where a fish has snatched an insect from the surface; mysterious little dimples that look like miniature whirlpools; the beaked head and long neck of a Texas softshell poked up like a periscope. This big turtle’s shell is really hard except along the edges, but it is smooth and doesn’t have the plates you see on other Rio Grande turtles. The Big Bend slider feeds primarily on plants, while the yellow mud turtle enjoys water insect larvae. And ready to oblige is a cloud of mayflies whirling in mad nuptial flight a meter or two (3-7 feet) above the water. They only live one day and exist as adults simply to mate, but they will sow the river with numberless eggs.
As you round a bend, a pair of great blue herons lifts from the shallows where they’ve been standing stilt-legged. Now with necks folded and long legs dangling they flap across to the farther shore. Ahead of you a blue-winged teal keeps lifting and settling further downstream. Ducks are seldom seen on the river in summer, but a dozen different species put down as migrants, and some even winter on the river. Now a slim pair of inca doves crosses overhead; you see the flash of rufous wings and white tail feathers. On a sandbar stands a spotted sandpiper, head low and tail high. He takes a step, stops, teeters up and down, and then flies.
The sandbar itself snugs in against the cliffside with greenery growing in three distinct tiers. River cane and mature salt cedar stand 4.5 meters (15 feet) tall against the flagstones. And stairstepped in front of these are seepwillows—not a willow at all, but a kind of sunflower that pioneers sandbars—and a younger, shorter stand of salt cedar. The canebrakes fairly crackle with wintering birds: black phoebes, cardinals, brown-headed cowbirds, and a migrating yellow warbler, perhaps. There is plenty for them to eat in that thicket. You yourself discover a nursery of orange true bugs beautifully crossed with olive green, all crowded together in every stage of development on two or three willow leaves. But the most intriguing thing about that sandbar is the record left by its visitors: a lizard’s five-toed track with the long unbroken mark made by its tail, and the great blue heron’s left-and-right footprints striding along almost in a straight line. You find the cat-like tracks of the ringtail, the dog-like tracks of the gray fox, and the flat-footed print of the hog-nosed skunk, pear-shaped as a bear’s. At the water’s edge honey bees are collecting moisture to water-cool their hive. One by one they sip and lift off, making a beeline for a cliff.
In the canyons where water flows from wall to wall, you find shore life restricted to those few plants and animals that can make a home on a cliff face. A spindly tamarisk has established a roothold in a thimble-sized deposit of soil just above waterline, and in cracks and crevices higher up, ocotillo and pricklypear are working down from the desert that tops the wall. Empty cliff swallow nests cluster on the undersides of overhangs. In spring you might see baby birds poking their heads from the colony’s doorways.
Along the border, people call the Rio Grande by its Mexican name, Rio Bravo del Norte. Nowhere does the river seem more wild, more powerful than inside Santa Elena, Mariscal, and Boquillas Canyons. To enter one of these mighty limestone vaults is to understand why mankind has always had to skirt canyon country, and why to this day, except for its historic fords, the river is all but impassable. If you go in by boat the only way out is through. The adventure calls for preparation, knowledge, hardiness, and considerable skill.
Cliff swallows colonize with as many as 50 nests in close order. How the adult birds pick out their own condominium from among such clusters remains a mystery.
But even a landlubber can stand in the canyon’s primeval presence. All you must do is make it up the ramps and steps that climb the cliff face at the mouth of Santa Elena, then follow the foot trail down again into the canyon. Looking up from the base of these 450-meter (1,500-foot) walls, you see a vulture and a raven soaring side by side along the canyon’s rim. To them you must seem small and as foolishly occupied as the ants drawn up in opposing lines across the sandy path. One step and you could crush the horde. One rock fallen from that height and you are gone. One wild storm upstream and you, the ants, and the sandbar are all washed away forever. Yet you are somehow drawn farther and deeper into the canyon, into this jungle of dark green tamarisk and emerald bermuda grass, through this labyrinth of water-polished boulders, to land’s end and water’s edge, to the very Beginning that laid these fossil oyster shells in this fierce rock.
Here in the canyon’s deep, vault-like isolation the sense of time, that ominous, inhuman distance of the Earth’s past, may come over you as the imagined shadow of the wings of a prehistoric reptile, the Pterosaur, perhaps, from 65 million years ago. This was the biggest flying animal ever known to have lived. Picture a 70-kilo (150-pound) flying reptile with a wing spread of up to 11 meters (36 feet), a foolishly long neck, and large head with a long, slender, toothless jaw. Add long legs, and short toes armed with sharp, hooked claws, and a body covered with fur-like material. And figure that each of those long, narrow, glider-type wings was a thin membrane supported by a single overgrown finger, and attached to the body, bat-style, right down to the knee. How could such a huge, ungainly thing ever lift off or fly?
A species does not survive unless it can compete for food and escape its predators, and Pterosaurs, both large and small, existed alongside aggressive, meat-eating dinosaurs for 140 million years. Unlike some smaller species found elsewhere, the Big Bend Pterosaur does not seem to have fished the ocean. At that time, Big Bend offered a river and floodplain environment far from the sea. No one knows how this giant Pterosaur made its living.
How the Canyons Were Formed
Some 200 million years ago this region lay under a sea whose sediments formed the structural, limestone bedrock patterns of the Big Bend. The basic landscape configurations of today’s park were set in motion 75 to 100 million years ago as the landscape emerged, folded, and faulted. Then erosion set in.
The ancestral river that carved Santa Elena, Mariscal, and Boquillas Canyons through bedrock was the Rio Conchos. (The present Rio Conchos contributes most of the water flowing through today’s Big Bend. It flows into the Rio Grande just upstream of the park.) When the ancestral river hit the limestone mountain uplifts, it had no alternative but to cut its way through. Steep-walled, narrow canyons resulted. Santa Elena Canyon is cut through the Mesa de Anguila. Mariscal Canyon severs its namesake mountains. Boquillas Canyon, the longest, cuts through the massive limestone Sierra del Carmen. You can see how steep these canyons are by taking a river trip (see [page 122]) or hiking park trails (see [page 119]) to the river or to canyon rims.
The region was once much higher in elevation than it is today, but erosion has taken its toll. Mountains and mesas are landscape formations whose rock erodes more slowly than surrounding materials do. Castle-like peaks and high, sharp-rimmed mesas stand as weathered monuments to earlier times when elevations were higher. Such stranded vestiges of geologic eras punctuate the stark Chihuahuan Desert landscape with eerie architecture. Astronauts have used Big Bend terrain to simulate moonscapes.
The sequence of geologic diagrams shows how the Big Bend canyons formed and what their future would be if slow processes of erosion continue.
① Faulting uplifts bedrock to form the mountain mass.
② Streams erode the mountains and begin to deposit sediments in the valley.
③ Streams continue depositing valley sediments eroded from the mountain mass.
④ Streams have now cut clear through the sharply eroded mountains and formed steep-walled canyons.
⑤ All but isolated mesa remnants of the mountains have eroded and weathered away. The riverbed rests in the deep layer of sediments.
⑥ In a future stage the bedrock mountain uplift might be entirely eroded away to become a deep layer of sediments.
When you think that the Earth is perhaps 4.5 billion years old, that complex organisms have existed for no more than about 700 million years, that the oldest rocks exposed in the park are 300 million years old, and that fossils of backboned animals in the park cover a time span of 70 million years, you can realize how fragmentary the fossil record really is. You get few glimpses of the relatively recent past, but these are astonishing.
For example, geologists know that back in the dim dark distances of Earth time, Big Bend lay repeatedly at the bottom of the sea. Convulsions within the Earth repeatedly raised these sea floors to the tops of mountains, and time after time these mountains wore away. One of the ancient ocean beds can be seen at Persimmon Gap where, in remnants of the park’s oldest mountains, fossil sponges, brachiopods, and other simple marine organisms lie exposed. For hundreds of millions of years the three-lobed trilobite was among the most prolific animals in the world, but it had long been extinct when the last great ocean, the so-called Cretaceous sea, washed across Big Bend. Most of Mexico lay submerged and a sort of mid-continent seaway cut North America in two about on the line of the Rocky Mountains. In its early stages this sea harbored ancestral clams, oysters, snails, corals, and a coiled shellfish called an ammonite. You can see these animals preserved in the limestone walls of Santa Elena Canyon. Giant clam shells a meter (3 feet) across and fossil fishes preserved in the round between Boquillas and Mariscal Mountain tell us what lived in later seas. Sea turtles, sharks, and a 9-meter (30-foot) marine lizard that swam in the open ocean have left their remains in the yellowish badlands near the park’s western entrance.
Late in this oceanic period the Rocky Mountains began to rise to the north of Big Bend, the Sierra Madre to the south. The park’s own Santiago, del Carmen, and Mariscal ranges, and the first upward thrust of the Chisos also occurred at this time. As the mountains rose and started to wear away, delta deposits began to build out farther and farther into the seaway, forming barrier bars and tidal shelves where turtles, snails, oysters, and sharks lived and died. Gradually the near-shore, subtidal environment changed to a tidal flat. This in turn changed to marsh, to beach, to brackish and freshwater lagoons, and finally to an estuary and river floodplain environment. Such was the Big Bend world to which the dinosaurs came. They had been ruling the Earth for eons, but they did not reach the park until the Cretaceous sea withdrew.
Ammonite impressions (top) and fossil clams reveal the era in which the Big Bend area repeatedly lay beneath a shallow sea. Fossil finds show that today’s mountaintops were once sea bottoms.
Sloshing about in the freshwater were amphibious and semi-aquatic species. Probably commonest was the duckbill dinosaur, an enormous reptile that walked on huge hind legs. It had as many as 2,000 teeth. Specially adapted for grubbing up and munching freshwater plants, these flat grinders occurred in batteries in the duckbill’s jaw. As a tooth wore out, another popped into place.
Tramping about on all fours and peaceably cropping land plants were bizarre looking horned dinosaurs with turtle-like beaks. For their weight they must have had the most powerful jaws of any backboned animal that ever lived. Armored species included one that looked like a horned toad the size of a dump truck. Another sported a huge lump of bone on the end of its tail, still another a thick lump of bone above a brain no bigger than the end of your little finger. The world’s largest crocodile, a 15-meter (50-foot) creature with 15-centimeter (6-inch) teeth, turned up in the park, but remarkably few bones of meat-eating dinosaurs have ever been found. One of the Earth’s great mysteries is why, at the top of their terrific form, the dominant dinosaurs died out? This happened in a geologically short time all around the globe. Perhaps it was because the highly specialized reptiles couldn’t cope with changes in their environment as the world climate grew cooler and more continental. No one really knows.
When the Age of Mammals began some 64 million years ago, Big Bend lay on an alluvial floodplain where summers were moist and winters were mild. Shallow rivers meandered between natural levees wooded with flowering plants, sycamore, stinking cedar, and tree ferns. Garfish swam in rivers and isolated ponds, while small lakes and swamps lingered on in meander cutoffs and abandoned river channels. It was a land where seasonal floods alternated with dry periods and where, during protracted dry spells, treeless areas invaded the forest.
The Fossil Record
Garfish and turtles in the Rio Grande give us a hint about life here 50 million years ago. Then they swam in waters haunted by crocodiles and visited by the modern horse’s earliest ancestor, Hyracotherium; the hippo-like Coryhodon; and Phenacodus, an early species of ungulate related to both hoofed and clawed mammals.
Fossils of these and other animals belie dry and barren Tornillo Flat’s earlier Eocene life as a lushly vegetated landscape, as depicted in this reconstruction.
At the time of this scene the dinosaurs (“terrible lizards”) had been extinct for 15 million years. Their fossils occur here too. Remains of the giant Pterosaur have been found. These “winged lizards” were flying reptiles whose 11-meter (36-foot) wingspan exceeded that of small jet fighters. The wing was a featherless membrane stretched out from the reptile’s body to the tip of its greatly enlarged fourth digit. Fossil remains of Brontosaurus, Allosaurus, Icthyosaur, and others have been found. Fossil Ammonites (see [page 67]), related to today’s sea-dwelling chambered nautilus, represent the even earlier period when today’s Big Bend was covered by a shallow inland sea. Sample fossils are displayed in a shelter off the Marathon entrance road near Tornillo Creek bridge.
With such a wealth of habitats a whole new host of animals took over Big Bend. Crocodiles and turtles hung on from the Age of Reptiles, but nature’s evolutionary torch passed to the warm-blooded mammals who increased rapidly in numbers, size, and diversity. Remains of 29 species of early, extinct forest-dwelling mammals have been discovered near the Fossil Bone Exhibit site on Tornillo Flat. This same floodplain later accommodated a hippo-like plant-eater, a browsing collie-sized mammal, a panther-like cat, and the little ancestral horse, Eohippus. No bigger than a fox terrier, Eohippus had not yet developed the typical horse hoof and still had four toes on his front feet and three on his hind. He browsed among low forest plants, because nature hadn’t yet invented grass.
Some 20 million years ago, when we get our next glimpse of the Big Bend, grasses were well established and the Earth began to witness the rapid rise of grazing animals. In the park, Castolon had a savanna-type environment, with a sub-humid to semi-arid climate. Rabbits and camel- and sheep-like mammals flourished. Plant eaters ranged in size from a tiny mouse to an enormous rhinoceros. This giant was about 3 meters (10 feet) long, stood 2 meters (7 feet) high at the shoulder, and had massive, bony horns on its head. There were also carnivores to fatten on the herbivores.
In the middle of the Age of Mammals, Big Bend country became the seat of widespread and repeated volcanic disturbances, with lava flows, ash falls, and mountains bulging up like blisters as they filled with molten rock. Here, most of these events centered on the Chisos, where the signs can be seen in mountain peaks to this day. Much of what happened since has not even left a shadow; the record and the rocks have both been erased by millions of years of weathering and erosion. But you can see “living fossils” in the high Chisos canyons. These are the ponderosa pines, Arizona cypress, and Douglas-fir trees descended from the moist woodland species that populated this region during the last Ice Age. Occasional remains of the great Ice Age mammoths have also come from gravels in deeply eroded ravines. And you can see all around you the evidence of two latecomers who seem to have reached Big Bend around the same time: Man and the Desert. But here, deep in the river’s canyon, both are apparently as remote from you as your own daily world.
University of Texas paleontologist Wann Langston excavates the sacrum of an extinct sauropod dinosaur. These bones were excavated near Tornillo Creek.
This huge cottonwood tree casts its shade along Terlingua Creek. Early settlers of Big Bend greenbelts used the cottonwood for roofbeams. Later, the trunks shored up mine shafts.