Marble Mountain Images

[Ursula Illiano]

TheKlamath group of terranes has a long and complicated history. In a blog post about my local region around Ashland, Oregon, I provided a brief introduction to these mountains: -revealed.net/the-rogue-valley-region-in-sw-oregon-displaying-300-million-years-of-geologic-time/. Basically, the Klamaths consist of a vast variety of oceanic materials, including all parts of the oceanic crust and overlying marine sediments. These oceanic pieces were accreted to each other and then to the continent via a long history of subduction that included colliding volcanic islands.

Before it was metamorphosed, marble was limestone. Shells of organisms on the seafloor created limestone rock overlying oceanic crust formed by volcanic (magma erupted on the seafloor) and plutonic (magma cooled beneath the seafloor) processes. Because of the high metamorphic grade, very few fossils are preserved. The limestone probably formed in a reef around a volcanic island that was located closer to the equator and that traveled eastward and northward with an oceanic tectonic plate. When the plate encountered a subduction zone, pieces of the plate and the overlying sedimentary rock were accreted to the continent through a complicated series of collisions.

Visit a classic fossil site on California's Mojave Desert--the specific place that first inspired my life-long interest in paleontology. The locality now lies within the Mojave Trails National Monument (officially authorized and established on February 12, 2016), where the BLM (Bureau of Land Management) continues to allow the casual collection of reasonable amounts of common invertebrate fossils--a designation that here includes trilobites; for fossil sites situated on private property, visitors must secure explicit permission from the land owners.

The beckoning dirt trail (at just left of center) leads slightly east of due north to the southern end of the Marble Mountains (tallest peak in upper center), Mojave Desert, California, wherein lies one of the most famous fossil localities in all the world--the classic trilobite quarry in the lower Cambrian Latham Shale (roughly 515 million years old). It's a place I first visited as a child with my parents on a weekend camping excursion--the very experience that inspired my life-long fascinattion with paleontology.

The Marble Mountains trilobite locality presently lies within Mojave Trails National Monument (officially authorized and established on February 12, 2016), where the BLM (Bureau of Land Management) continues to allow the casual collection of reasonable amounts of common invertebrate fossils--a designation that here includes trilobites; for fossil sites situated on private property, visitors must secure explicit permission from the land owners.

And now for the preliminary obligatory words of caution. Endemic to the Mojave Desert of California, including the Las Vegas, Nevada, region 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. 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 prospector who chooses to visit the Mojave Desert must be fully aware of the risks involved.

There are quite a number of popular rockhounding and fossil-bearing areas on the vast Mojave Desert, but one site in particular consistently attracts a great deal of attention. This is the classic Marble Mountains fossil quarry presently situated in Mojave Trails National Monument, San Bernardino County, California (officially authorized and established on February 12, 2016), where the BLM (Bureau of Land Management) continues to allow the casual collection of reasonable amounts of common invertebrate fossils--a designation that here includes trilobites; for fossil sites that occur on private property, visitors must first secure explicit permission from the land owners.

Here in the Marble Mountains abundant and well-preserved fossil trilobites can be found dating from the early Cambrian geologic age, or roughly 518 million years old--some of Earth's most ancient identifiable animals with hard parts--those wonderful arthropod trilobites that as a group survived for nearly 300 million years before their eventual extinction just prior to the rise of the dinosaurs some 245 million years ago.

The fossil trilobites in the Marble Mountains occur in a greenish to rusty-brown, platy-weathering shale called the Latham Shale, a detrital rock formation dated as lower Cambrian on the geologic time scale, or roughly 518 million years old. The Latham was named in 1954 by geologist John C. Hazzard for its excellent exposures on the western slopes of the Providence Mountains near an old and famous miner's cabin approximately 40 miles north of the Marble Mountains site--a specific place within the Providence range that is now off-limits to unauthorized collectors due to its inclusion in a federally protected wilderness area (one requires a special Bureau Of Land Management permit in order to collect legally within a federally administered wilderness region). Throughout its "type locality," in the region around the cabin where it was first described in the geologic literature, the formation is at least 60 feet thick and contains an abundant fauna of early Cambrian trilobites and brachiopods. At the classic trilobite quarry in the Marble Mountains, the Latham Shale averages around 50 feet in thickness and is also loaded with fossilized carapaces of trilobites, brachiopods, a siliceous sponge, a soft-bodied coelenterate (perhaps a jelly fish of some sort), an echinoderm and a mollusk or two. Virtually all of the trilobite specimens found at the old quarry--and in other exposures of the Latham outside wilderness boundaries, as well--were fragmental, although a few extraordinarily fortunate individuals reported that a whole, perfect fossil popped out at them from the shales.

The main reason there are so few complete, intact trilobite specimens to be found in the Latham--and at other early Cambrian sites, for that matter--has to do with the original fragility of the animal's exoskeleton. In actual life, trilobites possessed a thin outer covering composed of chitin--a hard, horny substance protecting the delicate soft-bodied organism within. While this material can be preserved in the rocks for millions of years, the problem is that the primitive early Cambrian trilobites--among the earliest known animals with hard parts--had loosely attached body segments. Thus, the head, thorax (middle portion) and tail tended to separate very easily upon the animal's death. Also, trilobites molted throughout their lives, periodically shedding their chitinous external covering in much the same way their modern-day relatives, insects, crabs, scorpions and pill bugs regularly shed their own exoskeletons during the molting process. The result was that the trauma of the molting often caused the already free-moving body segments of the trilobite to disassociate and break off, to be scattered by the sea currents.

All the trilobites found in the Latham Shale belong to a single significant family of trilobites called Olenellidae, or as they are more commonly called, olenellids. These were marvelously specialized arthropods, well adapted--for a time, anyway--to their life of burrowing in the muds of the shallow marine shelves along the margins of the early Cambrian land masses. Perhaps their most fascinating feature was their set of compound eyes, consisting of numerous minute calcite crystals in a closely packed arrangement. called scientifically a holochroal eye. It's not known for certain just how clearly the earliest trilobites were able to focus these eyes, but there is little question that, at the very least, they were able to distinguish adequately between predator and prey in their marine habitat. Mysteriously, though, the olenellids never survived beyond the early Cambrian, roughly 513 million years ago. Why they became extinct has never been fully explained, although the most logical idea is that, ultimately, they were ill-equipped to wage a successful struggle for life in their increasingly competitive environment. The Cambrian Period had ushered in the Paleozoic Era--the first occurrence of abundant complex animal life on Earth. It was a time of explosive biological diversification, and numerous burgeoning plants and animals were vying for every available ecological niche. Perhaps the olenellids, generally recognized by paleontologists as the earliest family of trilobites to appear in the geological record, could not overcome the inevitable encroachment of other increasingly aggressive varieties of trilobites into their domain. As a group, though, trilobites made it past the Cambrian Period and went on to flourish for close to another 300 million years or so, until at last they became extinct at the conclusion of the Permian Period, around 245 million years ago.

Roughly 280 million years earlier, though, trilobites flourished, and their remains have been found on practically every continent on Earth. In the western United States--in California and Nevada, specifically--most well-known trilobite localities occur in early Cambrian through middle Ordovician-age rock deposits. And, of course, one of the most famous places in California to find early Cambrian trilobites was at the prime fossil quarry in the Marble Mountains. There, the oldest geologic rock formation exposed is the early Cambrian Zabriskie Quartzite. This is a massive, resistant accumulation of heat and pressure-altered sandstone, mostly unfossiliferous except for a stray olenellid or two, plus distinctive vertical tubes that paleontologists call Skolithos. These represent the feeding burrows and living chambers of some kind of ancient, extinct suspension feeder. Sitting directly atop the Zabriskie--in bold, dramatic stratigraphic contact--is the fabulous early Cambrian Latham Shale, the specific rock deposit which contained a profusion of trilobite remains at the fossil quarry in the Marble Mountains; it's approximately 50 feet thick here, consisting of rusty-red to greenish-colored shale that typically weathers to recessive slopes, often masked by eroding debris from the overlying, younger rock formations in the local stratigraphic section.

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