I’ve always been fond of pebbles and cobbles, especially ones that are well rounded and polished.  They’ve travelled long distances, in rivers or maybe back and forth along beaches, in nature’s rock tumbler, rolling and rounding for thousands or millions of years by the force of flowing water.  When I was growing up in Illinois, I’d hang out by the Fox River and a local excavating company.  The Fox had an immense assortment of rounded rocks, carried downstream from their original sites in Wisconsin, Minnesota, or farther north in Canada.  They were transported by the ancestral Fox as melting glaciers sent torrents of water southward some 8,000 years ago.  The excavating company had mountains of gravel for local construction projects, piles of rounded granite, gneiss, quartzite, and an assortment of sedimentary rocks dredged from a nearby gravel pit.  Those local summits were only a block from my house, a short walk or bike ride by an amateur geologist. 

Years later, after pursuing my career ambitions to become a geologist, I investigated Cretaceous conglomerates (ancient gravels) to better understand ancestral rivers, and to determine their original sources far to the west in what is now Idaho.  Those gravels had suffered multiple cycles of transport and deposition by the time they arrived in Montana several million years later.  Long range transport takes a toll on rocks.  Limestone and shale could never survive a 200-mile trip.  They’re just too soft.  Surviving pebbles and cobbles are durable, mostly quartz (7.5 on the Mohs Hardness Scale) and maybe even some feldspar (6.5).  

The Earth’s crust is full of quartz, and it goes by many different names: silica, quartzite, chert, chalcedony, agate, opal, flint, and more.  There’s milky quartz, smoky quartz, amethyst, and citrine.  Quartz can form in volcanic rocks as black glassy obsidian, in deeply buried batholiths as large white crystals or veins, or as quartzite in high pressure-temperature metamorphic environments.  But it’s all just SiO2 with some impurities thrown in for good measure.  Create a mountain range, start eroding it, and send the debris downstream.  Much later, polished pebbles and cobbles (that geologists call clasts) are preserved in conglomerate beds for harvesting by lithic enthusiasts.  I once found a Cretaceous stream deposit comprised entirely of red, rounded and polished quartzite cobbles up to 10 inches in diameter.  Those magnificent clasts were falling out of a steep hillside exposure along a modern-day creek, coincidently named Rock Creek.  An old geologist friend and former state geologist in Montana said they were derived from the billion-year-old Belt Supergroup.  The Belt is a 50,000-foot-thick hodge-podge of metamorphosed sedimentary rock originally laid down as coastal sediment along the western shores of ancestral North America.

I’m hiking up a dry creek bed at Toadstool Geologic Park today.  Toadstool sits in northwestern Nebraska in badlands country, a varied terrane of receding spires, buttes and short grass prairies, home to meadowlarks, coyotes, antelope, and a few drifters in campervans.  But it’s getting muddy as I continue hiking upstream.  The area received an inch of rain less a week ago, the result of strong, early summer storms that hatched deadly tornadoes farther east in central Nebraska and Iowa.  The recent high-water line is two feet above the creek bed.   My boots are getting heavy as the clay rich muck starts to stick.  I look up at the canyon wall, mostly clay-rich mudstone.  I’m walking in modern day mud recycled from the canyon wall, itself an accumulation of much older mud from mudflats in ancestral Nebraska. 

The name Toadstool originates from the park’s erosional features, rock pedestals of resistant sandstone blocks resting on mudstone.  The structures simply resemble large mushrooms.  These features are part of the Chadron Formation, an Eocene aged (35 million-year-old) sedimentary rock sequence comprised of mudstone interbedded with sandstone and conglomerate lenses, a vast alluvial plain of meandering rivers and mudflats draining the ancestral Rocky Mountains and Black Hills.  The Chadron alluvial plain was home to a diverse assemblage of extinct mammals, ancestors to modern day camels, pigs, horses, hippos, and even birds.  I linger at some cliffside exposures with thin conglomerate lenses. The grains barely fit the definition of pebbles in geological terms, 4-64 millimeters in diameter. My aging eyes need a magnifier to identify them. The lenses represent brief periods of higher energy stream flow, maybe storms or just Spring runoff.

I’ve been at the 6-site Toadstool campground for two nights, a brief visit to experience the geology, colorful sunsets, local fauna and flora, and solitude.  It’s usually a quiet place unless a storm is brewing, but it’s getting crowded today as local Nebraskans enjoy the Memorial Day weekend.  Visitors seek the natural beauty and show their children the abundant fossil bones and tracks of the extinct creatures. Toadstool is part of the much larger Oglala National Grassland, a 94,000-acre prairie parcel administered by Nebraska National Forest.  I leave the park, drive north on Toadstool Road, and after a few miles turn right on Sugarloaf Road.  Sugarloaf is named for a prominent haystack butte, an isolated 300-foot Chadron monolith south of the road.  It’s breezy and cool today, and the skies are clear as I drive up, onto a highland surface of hills and swales. 

I’m looking for a  new site to camp, an area with extensive views, away from humanity.  I soon see a sign for Boardgate Reservoir and decide take a short side trip, but the route to the reservoir is deeply rutted.  I park my van along Sugarloaf Road and walk downhill on the graveled road, about a mile.  I inspect a conical hill of brown and yellow Chadron mudstone along the way, a likely place for mammal bones.  A hardpan desert pavement around the base is covered by a veneer of multicolored pebbles and cobbles and shattered ironstone concretions.  The red-brown ironstone suggests some sort of fossil soil horizon in the Chadron.  I suspect I’m lower (and older) in the Chadron Formation than at Toadstool.  I just stand and stare at the colorful art installation for a few minutes, but I sense something confusing or maybe contradictory.  Where are the pebbles coming from?  I inspect some glossy specimens in shades of white, yellow, red, and gray.  But I’m soon distracted.  I see a 3-inch Paleoindian scraper, a Stone Age tool of yellow chert, probably dropped or discarded by a nomadic hunter.  It’s impossible to identify the age of a lithic like this unless there’s some additional context, maybe a unique construction style, other associated artifacts, associated trade items.  I return the scraper to its home and complete my circuit around the hill.  I spot several lead slugs, probably left by locals getting some target practice, and then a quarter, a recently minted one in fact, maybe dropped by one of the shooters.  My lucky day.

I resume my walk to Boardgate and identify some ducks, a blue winged teal, some coots, and a mallard.  There are several aggressive killdeer in the air around me.  I’m invading their territory.  They may be nesting and don’t appreciate my visit.  They fly overhead and scream relentlessly with their loud and persistent calls: deeeyee, cheeedeer and bereedddeew, but it sounds more like cheeese bergger to me.  Their distress calls are informing me it’s lunch time, so I return to my van and indulge in cheese cubes, crackers, and apple slices. 

I continue driving until I notice a weathered plastic marker and a two-track roadway heading south.  I slowly drive for a half mile and park nearby and survey my grassy pebbly surroundings.  Sugarloaf Butte and the BNSF railroad tracks lie far to the west.  I’m 10 miles south of the South Dakota state line.  The nearest gas station is at least 25 miles away in any direction.  I’ve chosen an ideal spot, a perfect home, not a human for miles, just some happy Herefords in the distance munching on the endless pasture and the muted rattle of an empty coal train heading to Gillette, Wyoming.

 I look down at the hard cobblestone surface under my feet.  I’m underlain by a veneer of rocks, a concoction of many sizes and colors, all rounded and polished, and tightly embedded in the brown mudstone soil.  The rocks are similar to those along Boardgate Road.  I suspect I’m standing on a modern-day pediment, a surface of erosion covered in gravel from countless streams and rivers flowing over the region during the last 5-10,000 years.  Pediments can form in front of mountain ranges or even much farther away, especially in arid regions where chemical weathering plays only a minor role in disintegrating rocks.  I look at the ground again and then to the south and west with my binoculars.  The clasts are actually everywhere.  Continental glaciers never reached the Ogalala Grassland, but melting alpine glaciers in the Black Hills and Rocky Mountains sent debris laden meltwater in every direction. 

I focus my binoculars on an outcropping of dark gray shale a few miles northwest and close to a paved highway.  The rock appears much darker than the brown mudstone under my feet.  I suddenly realize my hypothesis regarding the gravels, the ironstone concretions, and the surface I’m standing are probably wrong!  My temporary home is secure, so I decide to investigate the dark shale.  It takes me 30 minutes to reach the outcropping by driving a few miles, first on my dirt track back to Sugarloaf Road, then to the paved highway, and finally north. I find my outcropping and park nearby a few hundred yards off the road. 

The dark shale forms the lower part of an exposed hillside overlain by yellow brown mudstone and the now familiar veneer of pebbles and cobbles.  I’m staring at the Pierre Shale. The Pierre (named for outcroppings at Pierre, South Dakota) represents the last inundation of marine waters in the western US.  The continental sea retreated some 70 million years ago.  The top of the Pierre became deeply eroded as mountains rose and rivers flowed through the region.  Soils began to form.  My graveled surface represents the winnowing and recycling of sediments millions of years ago, not the modern-day pediment I first thought, but a fossil pediment, sitting atop the eroded Pierre surface and before the Chadron mudflats formed.  I’m viewing processes related to the uplift of the Black Hills.  I walk the slopes and manage to avoid a tightly coiled rattlesnake taking an afternoon nap.  No rattle.  I’m standing on what geologists call an unconformity, an irregular-surface of erosion deep into the Pierre, capped by a thin gravel layer of well-worn clasts, and a fossil soil horizon comprised of yellow mudstone and shattered ironstone concretions.  This entire process did not take place 5-10,000 years ago, but during a 30-million-year interval after the retreat of the Pierre Sea! What’s more, several rock units including the Fox Hills, Hell Creek, and Fort Union Formations were deposited on top of the Pierre Shale in the region before the cycle of erosion. Those rocks are still exposed in northern South Dakota and eastern Montana, but were either never deposited or completely stripped from the Oglala landscape during that 30 million year period. Those now missing rocks totaled several thousand feet in thickness.

It’s 3 PM, plenty of time for a short hike and further investigations into pebbles and cobbles.  I return to my campsite, grab my rock hammer, and start walking south.  I scour the abandoned roadway and nearby hillsides for clasts.  I’m impressed by the range of rock types, black chert, red quartzite, brown metamorphosed volcanic rocks, white milky quartz.  I reach for a large pebble, a polished chunk of cemented gravel comprised of tiny pebbles of black chert and milky quartz.  I ponder the specimen, a pebble of tiny, much older cemented pebbles.  The range of clast types at the top of the Pierre suggests a complex geologic history all thrown together in a durable gravel medley.  But geologic processes are selective, and limestone and mudstone clasts are noticeably absent.  They would never survive the long arduous journey south from the Black Hills.  Nature has been mercilessly selective.

I inspect a quartzite pebble with conflicting laminations, suggesting strong opposing currents in the original sediment.  It was probably formed in a deeply buried Cambrian sandstone somewhere in The Hills.  Many of the clasts mystify me.  I once knew the rocks of western South Dakota well.  I was an undergraduate student at South Dakota School of Mines in Rapid City in the 1960s, but time has taken a toll on my geologic memory.  I’m baffled by the origin of yellow and orange cherty pebbles and numerous dull gray clasts.  The most abundant specimens are the most striking, milky white quartz clasts probably originating in granite pegmatites common to the northern Black Hills.  I lift a five incher and imagine its 1.7 billion year history, from a mass of Precambrian quartz, maybe a large quartz vein in a deeply buried granite, then to a rounded and polished cobble sitting in a gravel bed atop the 70-million-year-old Pierre Shale far to the south.

I have a creative idea—at least creative to me.  I often give geologically-oriented gifts to friends and relatives.  I’ve offered fossils, colorful pebbles, and oddly-shaped lithic curiosities to the unsuspecting masses.  I include a short geologic history and gift wrap my present.  Quartz pebbles make perfect gifts.  I’m thinking of Mark and Rex now, my brothers-in-law, both occasional rock afficionados.  Rex actually made a 50th anniversary pendant for his wife from gold he had panned along the banks of the Columbia River in Oregon.  I start collecting but soon decide to complete my shopping spree when I have sample bags to fill.  I soon find another artifact, a sharp-edged rectangular scraping tool carved from yellow chert.  

My avian neighbors deserve attention too.  I identify several meadowlarks, a lark bunting, some horned larks and lark sparrows, but no kildeer.  Lark seems to be the key word here.  I reach the top of a hill and notice a parked vehicle and two humans about a half mile ahead along an adjoining road.  I grab my binoculars.  Two women in jeans and knee pads are kneeling next to buckets of water, using picks or chisels to access half buried rocks.  They seem to be shopping too.  I’m curious.  Maybe it’s about archeology or attractive pebbles.  I walk the distance and notice their vehicle, a Dodge SUV with Colorado plates.  Not locals.  I hate to scare people, so I announce my presence well in advance.  We share greetings.  I then ask what they’re looking for.  “We’re trying to find Fairburn agates”, the younger woman volunteers.  “Fairburn agates”, she repeated. 

It’s like a slap in the face, a sudden shock, the realization that I’ve missed something important.  I’ve completely forgotten about Fairburn agates.  Agates are a type of cryptocrystalline (microscopic crystals) quartz with multicolored rainbow-like mineralized bands.  Aficionados cut and polish them.  The highest quality specimens are sold for hundreds of dollars.  Agates can form in sedimentary or volcanic rocks from percolating mineralized waters.  Fairburn agates, named for a tiny community along the east side of the Black Hills, are noted for their brilliant shades of red, orange, brown, and black banding.  They are relatively rare.

We chat for a few minutes, about agates in general and the hobby of collecting agates. The women are from Fort Collins, Colorado and travel to the Nebraska Panhandle frequently.  I wish them luck and begin my return trek.  It’s mostly uphill, so I walk slowly, focused on the gravel pavement, and the raw, rough outer surfaces of Fairburn agates.  I kick some pebbles and ponder my memory lapses today.  Maybe I’ll find an agate, or maybe not.  I soon see a shiny circular object in the late afternoon sun.  I look down and collect another newly minted quarter.