Visiting a Prairie Pompeii to See our Mid-Continent Region

     During my days as a geology instructor, I often saw the disappointment from students on field trips to view fossil excavations.  Having seen T-rex and triceratops skeletons embedded in their reconstructed environments at some of our great museums, students are bummed out to observe nothing more than bits of bone fragments and disembodied teeth in a sandy matrix.  The sad fact is that almost all fossil remains of exotic creatures from the past consist of disarticulated bones that have suffered a long tumble among gravel and boulders before collecting in the bottom of a lagoon.  Even more disappointing is the observation that most of the exposed landscape in the Ozarks has been eroding away for more than 200 million years.  The remaining rocks contain a few bivalve shells, fragments of crinoid stems, or the flattened stems of coal swamp plants – all from ages before large animals roamed the planet.  The only place we can see degraded remains of truly impressive creatures is in the far southwest of the state where sediments consist of a pro-grading coastal plain with some respectable dinosaur trackways.  A little to the northwest there are the remains of large sea-going predators like ichthyosaurus and plesiosaurs that sank into the mud beneath the shallow midcontinent seaway that persisted through much of the Mesozoic era.  Even then, the bodies most often drifted around and disintegrated before being permanently entombed.  But there is one place within a long day’s drive to see a scene where a Vesuvius-like eruption entombed a herd of animals in a similar display to what archaeologists find today in Pompeii.  Just as at that famed Italian site, we can see ancient life buried in volcanic ash to preserve articulated skeletons of creatures embedded at a single moment of time in the exact place where violent death overtook them.

    First, the identification of our American Vesuvius – the volcano that entombed a small version of our mid-continent landscape at a single moment in the deep past.  The blanket of volcanic ash that embeds the site came from one of the repeated eruptions of the great caldera that we now see in Yellowstone National Park.

     Eruptions from that location have been repeated at about half-million-year intervals.  These were gigantic events spewing enough pulverized rock to blanket a good part of western North America.  This is “hot spot” vulcanism in action.  The details are still controversial, but the most commonly cited description is of a large plume of super-heated lava generated at the boundary of the earth’s inner core.  This blob eventually becomes so buoyant that it rises towards the surface to generate a great expanse of lava flows.  One such vast lava field covers much of western Washington state – the Columbia Basalt Field.  The deep-seated pathway created by the rise of this great plume lets additional, smaller pods of hot lava drift upwards for an extended period afterwards.  This process is so slow that the hot lava emerges to form an eastward-trending line of craters as continental drift pushes our plate to the west.  We now see this as the Snake River Plain of southern Idaho with Yellowstone NP at its eastern tip.  There’s good news and bad news here.  The good news is that this massively destructive eruption occurs only every half million years or so.  The bad news is that the last one was about that long ago.

      Now for the American Pompeii part. This site is a display developed at Ashfall State Park in northeast Nebraska.  If you plan a trip, it’s about an hour northwest of Omaha.   The exposed floor of the site is located in a gigantic hangar-like building where you can see paleontologists at work exposing the bodies of a mixed herd of a rhinoceros-like creatures with a few horses and camels thrown in.   The sedimentary bed where the bones lie is dated at about twelve million years ago.  The center of eruption had then drifted to western Idaho – still a long way from its current Yellowstone location.  You can see nearly fifty of the fully articulated skeletons, some obviously mothers with half-grown offspring at their side.  The geological section shows that about a foot of ash had blanketed the landscape at a location almost a thousand miles from the eruption.  That must have been some blast.  At the time of the blast, this site was a depression in the flood plain of a river into which the blast deposited a blanket of sand and gravel from the mountains to the west (ancestral Platte River).  After the ash was deposited, wind erosion created a dune that was about ten feet in depth to encapsulate the site for future development of today’s state park.  Hence the analogy with the famed archaeological display in Italy. Even though this site is a long day’s drive from the Ozarks, it provides a truly rare insight into what the central part of the continent was like over the many millions of years when erosion was removing almost all evidence of life from its exposed surface.

       Paleobotany shows this site had a subtropical savannah climate, probably with seasonal rainy and dry periods – distinctly warmer than today with probably less seasonal temperature variation.

      Reconstruction of conditions at the time of their death shows the doomed rhinos huddling in a depression that probably contained some standing water.  Many are preserved in their death position with legs collapsed beneath them. The evidence shows that they did not die instantly.  Certain kinds of bone deterioration indicate that they died slowly from the effects of inhaling volcanic glass shards as lung function was progressively lost.       

     You can imagine seeing the struggling animals huddled there in a landscape where everything is now buried in maybe a foot of powdered volcanic glass.   The bones lower down at the base of the deposit include birds, turtles, and lighter-boned camels that show indications of damage from trampling.   This probably indicates that these other creatures died almost instantly, with the rhino herd coming in afterwards seeking water and lingering on for perhaps a few days until their damaged lungs gave out.  The underlying sediment layer of river gravel provides a contrasting record of bone fragments of a wider variety of savannah life (including four-tusker elephants and three-toed woodland horses) with their scattered bits of damaged bones presenting the kind of transported and deteriorated evidence we are used to seeing in most continental deposits.   Ashfall State Park may be located in Nebraska, but the site provides the best insight we have into what an entire lost era might have been like before the uppermost surface of the Ozark Plateau was washed away forever.