Scenes From The Loop Trail

Fossil Bone Basin, Mojave Desert, California

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At Fossil Bone Basin, the Bureau of Land Management administers a wonderful one-way scenic loop trail that penetrates some of the best-exposed outcrops of the middle Miocene Barstow Formation. Here, the dirt trail gradually carries the traveler toward wildly eroded, varicolored exposures of a predominantly sandstone-tuff sedimentary complex representing the Lower and Middle members of the Barstow Formation, deposited primarily in streams and rivers some 19.3 to 15 million years ago. Here occur several fossiliferous horizons that bear important suites of mineralized skeletal elements from many extinct mammals, plus beds of water-laid, re-worked tuff that reveal many trackways of extinct horses, camels, pronghorns and proboscideans.

Massive beds of sandstone tilt toward the south, along the north limb of a great downfolding of strata geologists call a syncline--sort of an inverted rainbow-shaped orientation of Barstow Formation sedimentary rocks, warped through geologic time by faulting and folding.

Here the one-way loop trail cuts through a narrows in the Middle Member of the middle Miocene Barstow Formation. Several undersides of volcanic tuff beds throughout this region reveal quality fossil tracks (called ichnofossils by sedimentologists and paleontologists) of extinct horses, camels, pronghorns and proboscideans, primarily. Rocks exposed here are roughly 16.3 to 15 million years old.

Probably one of the most frequently photographed geologic spectacles in Fossil Bone Basin, along the Loop Trail. Here a relatively thin bed of hardened volcanic tuff arcs around the hillslope, forming a curious horseshoe-like outcropping due to tortuous earth processes of faulting, folding and ceaseless abrasive erosion. Its surface exposure at far left, and far right, particularly, reveal many tracks of extinct mammals.

A dramatic, cathedral-like buttress of massive river and stream-deposited sandstones, topped by white volcanic tuff, along the one-way loop trail, in the Middle Member of the middle Miocene Barstow Formation. Fossil bones are not as abundant here as in the overlying, and hence younger, Upper Member of the Barstow, yet persistent paleontologists have identified several productive bone-bearing layers within the middle sequence--in addition to localities that yield petrified woods, diatoms (an aquatic single-celled plant), and fresh water mollusks.

Finer-grained mudstones and siltstones (lower slopes) grade into crazily folded massive sandstones above along the one-way loop trail through the Middle Member of the Middle Miocene Barstow Formation--here around 16 to 15 million years old.

Ridges capped by relatively thin beds of volcanic tuff tilt toward the north along the south limb of a great downwarping of strata geologists call a syncline--sort of a huge inverted rainbow-shaped structure of folded sedimentary strata. Some of those tuff beds yield many tracks of extinct mammals.

Exposed along the lower slopes here are greenish-brown lacustrine (lake-deposited) mudstones and siltstones reminiscent of bone-bearing beds higher in the geologic section, in the Upper Member of the middle Miocene Barstow Formation--yet, these exposures occur in the older Middle Member. Rocks in upper third of image reveal a more characteric massive fluviatile (river-deposited) sandstone and volcanic tuff lithology characteristic of the Middle Member.

One encounters several "narrows" along the one-way loop trail through the Middle Member of the middle Miocene Barstow Formation, Fossil Bone Basin, Mojave Desert, California. Here, the traveler gains a close look at one of the massive river-deposited sandstone beds in the Barstow Formation.

Lots of excellent exposures of river-laid sandstones and volcanic tuff (whitish bed at skyline, upper center of image--and brownish ledge at upper right) along the one-way loop trail. Dirt trail can be seen at lower left. At this point in the journey, where the road narrows without access to a safety turnout, one really hopes that somebody up ahead is not headed illegally in the wrong direction.

Barstow Formation sandtones and volcanic tuffs dip toward the south along the north limb of a great downwarping of strata geologists call a syncline--a great reversed rainbow of sedimentary layers folded through geologic time. Here, the rocks date to approximately 16.3 to 15 million years old.

Return To Fossil Insects And Bones On The Mojave Desert, California