| Click the image below for a larger picture. The view from the crest of California's Inyo Mountains, near the Upper Mississippian Chainman Shale ammonoid locality, includes spectacular panoramas of the Sierra Nevada. Mount Whitney, the highest point in the contiguous United States at 14,495', is the peak just left of center along the Sierran skyline--a small white cloud touches its tip. Strata in the foreground include the Upper Mississippian Chainman Shale, which yields many species of ammonoids and pelecypods, in addition to terrestrial plants and even shark teeth at a unique fossil locality that lies in the vicinity of the old ghost town of Cerro Gordo, Inyo County, California. |
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An unusual fossil locality occurs east of Owens Lake, near the crest of California's Inyo Mountains--a place many fossil enthusiasts call the Chainman Shale site, where 325-million-year-old ammonoids can be found along the same bedding planes that yield fossil shark teeth and terrestrial plants. The fossil remains have been preserved in what geologists refer to as the Upper Mississippian Chainman Shale, a thick marine deposit, almost everywhere slightly metamorphosed, which also contains several species of pelecypods and brachiopods, in addition to a peculiar orthocone nautiloid cephalopod called Bactrites, or in more colloquial language the "darning needle" cephalopod because of its sharply elongated, needle-like appearance in the rocks. And now for the obligatory words of caution. Endemic to the Mojave Desert of California and southern Nevada (including Las Vegas, by the way) is Valley Fever. This is a potentially serious illness called, scientifically, Coccidioidomycosis, or "coccy" for short; it's caused by the inhalation of an infectious airborne fungus whose spores lie dormant in the uncultivated, harsh alkaline soils of the Mojave Desert; and the Chainman Shale site lies within a northern sector of the Mojave where Valley Fever spores have been detected. When an unsuspecting and susceptible individual breaths the spores into his or her lungs, the fungus springs to life, as it prefers the moist, dark recesses of the human lungs (cats, dogs, rodents and even snakes, among other vertebrates, are also susceptible to "coccy") to multiply and be happy. Most cases of active Valley Fever resemble a minor touch of the flu, though the majority of those exposed show absolutely no symptoms of any kind of illness; it is important to note, of course, that in rather rare instances Valley Fever can progress to a severe and serious infection, causing high fever, chills, unending fatigue, rapid weight loss, inflammation of the joints, meningitis, pneumonia and even death. Every fossil enthusiast who chooses to visit the Mojave Desert of California and southern Nevada must be fully aware of the risks involved. The Inyo Mountains fossil horizon lies in the vicinity of Cerro Gordo, an abandoned mining camp that produced millions of dollars' worth of silver, lead and zinc during the latter half of the 1800s. It is now a picturesque ghost town preserved in what is euphemistically termed a state of "arrested decay." Years ago, before the question of legal ownership of the property had been settled, the multi-hued, pulverulent mine tailings surrounding the town used to furnish collectors with such relatively uncommon mineral varieties as caledonite (a copper-lead sulfate), linarite (lead-copper sulfate) and leadhillite (a lead-sulfate-carbonate). But those days are now a distant memory in the minds of older mineral enthusiasts. Today, every last square inch of Cerro Gordo is privately owned, and mineral collecting within its posted boundaries is strictly forbidden without the owners' prior approval. For details about how to secure a permit to collect mineral specimens at Cerro Gordo, contact the regional office of the Bureau of Land Management in Ridgecrest, California. In the past, though, only "fully accredited individuals" have had success in finagling the essential legal documentation. Good luck. Not only is the Cerro Gordo fossil site a productive and scenic area to explore, it is also a place of great paleontological importance. As one of only three known Carboniferous (the European equivalent of the Mississippian and Pennsylvanian Periods combined) ammonoid localities in all of California, it is also the only one currently accessible to amateur fossil buffs. The other two occur in recently established (1994) Death Valley National Park, near the famous Racetrack in the northern sector of the vast park--now, with the assimilation of many thousands of acres of adjacent wilderness lands, larger than the entire state of Connecticut--where rocks of varying shapes and sizes apparently slide in mysterious secrecy across a wide desert playa. (The definitive explanation is that during winter, when nobody is around to see the phenomenon occur, preferably in the dead of night, fierce wind gusts--upwards of 100 miles per hour--push the rocks across slippery playa muds when they are saturated by rare episodes of Death Valley precipitation.) Both Death Valley sites are actually extensions of a single phenomenally productive cephalopod-bearing horizon in the Upper Mississippian Perdido Formation. They yield innumerable ammonoids that characteristically weather out whole and intact, although many of the cephalopodic remains reveal obvious signs of degradation to their exteriors caused by the ceaseless abrasive weathering in the harsh desert elements. Even so, numerous specimens still retain their original suture lines--that is, the distinctive growth line of the junction of a cephalopod's shell with the inner surface of its shell wall, which paleontologists use to identify the genus and species of all shell-bearing cephalopods, both living and extinct. Even though the Cerro Gordo locality fails to yield free-weathering specimens, its ammonoids and associated brachiopods, pelecypods, terrestrial plants and shark teeth are, nevertheless, common to abundant in the slightly metamorphosed detrital deposits of the Upper Mississippian Chainman Shale. They are preserved as attractive reddish-brown limonitic casts and molds of the original 325-million-year-old organisms, set on a matrix of pale-to medium-gray slaty shale. The majority of ammonoid specimens are rather tiny, with diameters of generally 10 millimeters or less. So, be sure to take a good-quality magnifying glass with you. Their preservation is fair to excellent--very surprising when you consider the degree of metamorphism the matrix was unavoidably subjected to in the geologic past. When the Sierra Nevada began to buckle upward during the late Jurassic Period, many relatively incompetent shale beds in the Inyo Range underwent moderate to severe alteration. That the ammonites and associated goodies in the Chainman Shale escaped this ravaging assault was indeed a miraculous occurrence.
It doesn't hurt to be in at least moderate physical condition, either. Once within striking distance of the fossil ammonoids and shark teeth, you will have to hike at elevations approaching 9,000 feet. For those unaccustomed to exertion at high altitudes, serious consequences can result--not the least of which is altitude sickness, a debilitating condition caused by prolonged oxygen depravation. The turnoff to the Chainman Shale fossil bonanza lies along the eastern side of Owens Lake--an essentially dry saline depression most of the year (occasional heavy runoff from the mountains during Spring sometimes results in a big shallow pond that evaporates quickly), near Keeler, where Cerro Gordo Road intersects State Route 136, 13.5 miles southeast of Highway 395. Check your pulse at this point and get a grip! The adventure begins at a modest 3,800 feet or so, with billows of irritating saline dust rising from "Owens Lake." Within just a few miles (when hairpin turns spiral upward and upward), you might reconsider having stayed behind in what had previously seemed the inhospitable Owens Lake below, and even find yourself obsessing on the flatness of it--that wonderful level expanse with no sheer drop-offs on either side. Turn east on Cerro Gordo Road. Here you leave civilization behind. You will be striking though a great wilderness: a geological wonderland comprised of thick rock deposits, in which many kinds of Paleozoic Era fossil remains can be recovered. These mainly include fusulinids, brachiopods, corals, bryozoans and crinoid fragments from the middle Pennsylvanian Keeler Canyon Formation and the Lower Permian Owens Valley Group. Also exposed in the area surrounding Cerro Gordo is the Lower Mississippian Tin Mountain Limestone (350 million years old)--a noted producer of corals and crinoids, in particular, from massive, reef-like carbonate accumulations in its youngest phases of sedimentary deposition. In the first 2.2 miles you pass through Pleistocene (roughly 1.8 to 10,000 years ago) to recent fanglomerate--extensive accumulations of eroded debris from every sedimentary and volcanic outcrop in the Inyo Mountains. Limestone cobbles in the alluvial material sometimes contain abundant fusulinid tests; however, because the host deposit consists of weathered rock out of its normal stratigraphic position, the best that can be said regarding its geologic age is that any fusulinid found within it probably came from either the Keeler Canyon Formation or the Owens Valley Group. These are the only rock units in the Inyo complex known to contain the distinctive wheat-shaped test secreted by an extinct single-celled animal. At that point, 2.2 miles from State Route 136, Cerro Gordo Road begins to slice through the Lower Permian Owens Valley Group, which is roughly 275 million years old. Here, the Owens Valley is composed of several sedimentary lithologies, including silty fusulinid-bearing limestone, lenticular organic limestone (within which brachiopods, corals, crinoids and bryozoans can be found), calcareous shales, sandy limestone, limestone-mud breccias, and relatively pure limestones. Fossil remains are not abundant in the Owens Valley exposures along Cerro Gordo Road. But farther southeast, in the Darwin District of Inyo County, profuse fusulinids and corals have been reported. For 1.4 miles Cerro Gordo Road passes through dramatic exposures of the Lower Permian Owens Valley Group. Then it intersects an unnamed terrestrial accumulation of Middle Triassic (220 million-year-old) volcanics and sedimentary rocks approximately 2,200 feet thick. The volcanic facies includes andesite flows, breccia and tuffs of gray, red and purple; among the sedimentary constituents are shale-sandstones and conglomerates of gray, red, green and purple. None of the land-laid Triassic exposures is fossiliferous, though. After cutting through the thick Middle Triassic terrestrial sequence for 1.5 miles, Cerro Gordo Road penetrates the marine Lower Triassic Union Wash Formation (roughly 235 million years old), some 1,800 feet thick. Unlike the classic outcrops at its type locality in Union Wash, northeast of Lone Pine, the exposures along Cerro Gordo Road bear only rare, fragmental ammonoids representing the genus Ussuria. The cephalopods occur in brownish-gray, silty limestones some 50 feet thick, along with abundant minute gastropod molds and infrequent pelecypodal lenses. The Union Wash Formation is wonderfully exposed for 0.8 mile, forming craggy, reef-like ridges and colorful slopes composed of thin-bedded shales in hues of pale greenish-gray, light gray, yellowish-orange and slightly greenish-yellow. At a point 5.9 miles from State Route 136, Cerro Gordo Road intersects the Middle Pennsylvanian Keeler Canyon Formation (295 million years old). It is approximately 2,200 feet thick and is predominantly a carbonate-shale sequence, in which the arenaceous to argillaceous limestones often yield abundant tiny fusulinids that are only moderately well preserved, for the most part, as well as a minor amount of crinoidal debris. Typically, the shale interbeds are totally barren of paleontology--yet, from a perspective of casual inspection, they seem so inviting, appearing eminently suitable for the preservation of many varieties of Paleozoic organisms. Persistent investigations of them may eventually reveal something truly remarkable. For the next 1.1 miles, the Keeler Canyon Formation outcrops in prominent fashion along both sides of the road, affording easily accessible exposures for fossil explorations. Abundant small fusulinids and occasional disarticulated crinoid stems occur at irregular intervals throughout the carbonate sequence. At a point 7.0 miles from State Route 136, the Middle Pennslyvanian strata rest in a prominent fault contact against the older Upper Mississippian Chainman Shale (about 325 million years old). The first Chainman outcrops encountered consist of smooth slopes underlain by dark gray to black carbonaceous shale and blocky-weathering argillite (a heat-and-pressure-altered clay shale), with subordinate interbeds of fine-grained sandstone and limestone. Periodic roadcuts during the next 1.3 miles--all the way up the remainder of the grade to Cerro Gordo Summit--reveal unfossiliferous brownish-red to dark gray argillites and characteristic thin-bedded, often cleavable, black shales.
The most abundant ammonoid represented at the Chainman Shale site is Cravenoceratoides nitiloides, a type originally described from a locality near Yorkshire, England. Less commonly observed species of ammonoids include Cravenoceras nevadense, Cravenoceras richardsonianum and Eumorphoceras bisulcatum.
Based on their distinctive suture patterns, all ammonoids and ammonites can be classified into three separate orders: goniatitic (species with nonserrated sutures, generally considered the most primitive varieties)--the kind found in the Chainman Shale; ceratitic (sutures with serrated lobes); and ammonitic (very complex suturing--usually referred to as the most advanced order of ammonites--and the only order that can properly be termed an ammonite; the goniatitic and ceratitic types are customarily called ammonoids). The goniatites first appear in the geologic record during the Devonian Period, some 370 million years ago; they persisted all the way up to the great dying at the conclusion of the Permian Period (when trilobites finally disappeared, as well), 248 million years ago. During the Permian Period both ammonoid and the ammonite varieties became common. But by Triassic times (248 to roughly 206 million years ago), only the ceratitic forms proved particularly successful. They, too, died out at the conclusion of that geologic period, leaving only the ammonitic types, the ammonites proper, to carry on the cephalopodal heritage. Throughout the Jurassic Period (195 to 150 million years ago) ammonitic ammonites thrived, becoming increasingly complex and numerous in the oceans of the Mesozoic world. They persisted right up to the close of the Cretaceous Period (65 million years ago), becoming extinct along with all the sensational, terrestrial giants of that age--the dinosaur. In addition to the cephalopods, the molluscan class Pelecypoda is well represented in the Chainman exposures. The pelecypods here are typically much larger finds, easily spotted as reddish-brown limonitic impressions and silvery sheens--original, lustrous shell material may be present in a few instances--on the darker, grayish shales. Two of the more common varieties present include Caneyella wapanachensis and Caneyella richardsoni, each of which is frequently found with both valves preserved intact.
Lying directly above the fossiliferous grayish shales is an inconspicuous three-foot layer of silty limestone. Thick talus overburder of weathered shales masks its presence, but a variety of Paleozoic invertebrate remains have been identified from this narrow horizon, including the corals Triplophyllites and Chaetetes, a fenestellid bryozoan, a trilobite (Proetus missouriensis), a gastropod (Pleurotomaria brazeriana) and the following brachiopods: Spirifer (two species), Composita lewisensis, Productus (two species), Diaphragmus elegans and Dictyoclostus sp.
While today the specimens lie at an altitude of roughly 9,000 feet, during Late Mississippian geologic times they would have been buried perhaps thousands of feet below sea level by detritus eroded away from already long-vanished mountains. Now, in the rarefied atmosphere of the high Inyo Mountains, fossil collectors become deep-sea divers of the Paleozoic Era, plunging far below the surface of the eons to explore layers of lithified muddy ooze, to search for ancient animal life that once took in oxygen from many fathoms below Earth's surface some 325 million years ago. |
Prior
to their having survived obliteration by powerful geologic processes,
the Chainman Shale organisms were deposited at the muddy bottom
of a rather deep, warm-water Paleozoic sea some 325 million years
ago. Now they lie at elevations of 8,800 to 9,000 feet near the
crest of the great Inyo Range near Cerro Gordo ghost town. The
ride up to the site, along the precipitous western flanks of
the Inyo Range, is a hair-raising adventure. The jeep trail climbs
over a mile in a mere seven or eight miles...although the vast
majority of that amazing ascent takes place within a distance
of only three or four miles! Needless to say, only those with
a reliable four-wheel-drive vehicle should consider accepting
the challenge.
The
fabulous Paleozoic Era fossils occur in grayish-black, slightly
fissile shales of the Upper Mississippian Chainman Shale at a
lone, isolated locality which lies in the vicinity of Cerro Gordo
Summit; while fossil specimens are quite common at the specific
site, preserved through roughly 50 feet of strata, collectors
will have to watch carefully for the invariably minute ammonoid
casts and molds, though fragments of mature specimens demonstrate
that the largest varieties grew here to approximately 60 millimeters
across.
Not only are invertebrate fossils common in the Chainman
outcrops, but infrequent 
Indeed, such brachiopods, bryozoans and corals all add
dramatically to the sensational plethora and variety of fossil
specimens that can be recovered from the Chainman Shale locality.
Here occur loads of nicely preserved ammonoids, nautiloid cephalopods,
pelecypods, gastropods, brachiopods, corals, bryozoans, terrestrial
plants and even shark teeth near the crest of the Inyo Mountains
in eastern California, in the vicinity of the ghost town of Cerro
Gordo.