Fossicking the Rockies
What was I thinking?
At 1 a.m. the night before Thanksgiving I’m freezing my ass off in a tent 8,500 feet up in the Rocky Mountains. Neither my old sleeping bag nor every available piece of clothing is keeping the cold at bay. It is impossible to fall asleep when your teeth won’t stop chattering. Was the prospect of fossil hunting worth this?
I had arrived in Denver a few days before. My family was convening at my brother Bill’s for the holiday, but I flew out early to do some exploring on my own. I had considered heading up to Dinosaur National Monument, but that was too far to get me back in time for turkey, so I decided to visit Florissant Fossil Beds instead. Bill moved to Colorado years before to attend the School of Mines in Golden. Though he’d long since quit geology for a career in finance he still gets excited about rocks. So when I told him my plan he ushered me out to the garage and rummaged until he dug up his old college crack hammer. He also lent me a dog-eared copy of Roadside Geology of Colorado and a tent. Thus equipped I drove south out of the Denver and into the past.
I-25 skirts the Front Range, so I had the mountains towering on my right and the plains stretching to the horizon on my left. Heading west off the highway at Colorado Springs I gained some 3000 feet in elevation as I climbed into the Rockies. Though Pike’s Peak loomed snowbound to the south the weather was balmier than expected, deceptively so. 100 miles from Denver, the town of Florissant is nothing more than a cluster of stores huddled around the intersection where I turned off for the park.
Florissant Fossil Beds National Monument is comprised of wide mountain meadows among hills covered with fir, pine and aspen. Back in the Eocene, volcanic mudflows buried an ancient redwood forest and dammed a lake into existence. Subsequent ashfalls buried and preserved the lakeside biota of 34-35 million years ago. Tens of thousands of specimens have been collected here since the 1800s. The visitor center displays a broad selection of butterflies, leaves, spiders, fish, birds and more, all fossilized in near photographic detail. More than 140 species of plants and over 1,400 species of insects have been identified, many foreign to modern Colorado or just plain extinct. A hike around a meadow brought me past giant petrified redwood stumps that would rival their living kin in California.
It is of course illegal to collect fossils within the monument, but the former lakebed deposits extend far beyond its boundaries. A ranger was happy to give me the phone number of a nearby ranch where I could prospect for a fee. It being late in the day, she also pointed me toward a campground down the road. As I set up Bill's tent beside a burbling stream the fact the campground was deserted should have told me something.
After supper in the eerily under-populated casino enclave of Cripple Creek I returned to my campsite. Before turning in I lingered gazing up at the vivid stars framed by the black peaks of the surrounding mountains. The temperature had begun to plummet as soon as the sun went down, but I’ve done my share of winter camping - on a windswept saddle of the Continental Divide, in waist deep snow on the North Rim of the Grand Canyon. There wasn’t even any snow on the ground here, I was sure I could handle it.
I was so wrong. I don’t know if it was my ancient sleeping bag, or that I had no companions to share body heat with, or maybe I had just got too old for this. I couldn’t sleep for shivering. Sometime after 1 a.m. I’d had it. I stuffed everything back in my rental car and drove 15 miles to Woodland Park, the nearest town with a motel. As I staggered through the automatic doors the night clerk gaped at me with shock and pity.
I felt immeasurably better after a night in a queen size bed. And I felt immeasurably smarter when I drew the curtains on a fresh blanket of snow, blinding in the morning sun. After breakfast I called the number the ranger had given me for the Florissant Fossil Quarry and made sure I was welcome to come by.
Florissant Fossil Quarry
The quarry is located on a ranch just outside Florissant center, right next door to the Monument. I was met by a free roaming chestnut mare, curious who this visitor was, followed by owner Nancy Anderson. Nancy led me across the snow patched meadow to a spot under the pine trees where shale strata were exposed in the side of a hill. I wasn’t allowed to dig into the formation itself, but there were tons of chunks of rock on the ground I could split. Nancy showed me the layer that held the fossils and how to expose them by slicing away thin sheets of shale with a razor blade. Then she left me to it. I chopped and shaved rock for an hour and half, finding lots of anonymous woody stalks and a few leaves. My best bit was a two inch serrated leaf from the extinct beech Fagopsis longifolia. I had opted to split rock on site for an hourly fee but Nancy also sold pre-packed sacks of shale to take away and split at leisure.
Over lunch I thumbed through Bill’s Roadside Geology and noted an entry for the promisingly monikered Baculite Mesa. I’d seen references to this place on the internet. The fossils date from the late Cretaceous, and as the name suggested, is famous for its cephalopods. RGC didn’t give exact directions to collecting sites, but did provide a crude map of its location outside Pueblo. That was just 50 miles further down the interstate, I thought I’d chance it.
Baculite Mesa is hard to miss, you can see it paralleling the highway to the east as you pass through Pueblo. Getting to it is another matter. After some driving around I found the turnoff on the outskirts of town: Baculite Mesa Road. I figured this must be the place. The asphalt quickly turned to gravel and “No Trespassing” signs began to crop up on either side of the road. There was a series of big wooden bullseye targets standing out amid the scrub and I began to worry I was wandering onto some kind of military firing range. As the road wound toward the foot of the mesa I ran into a bunch of trailers and construction equipment, and more No Trespassing signs. One of the signs had a phone number on it, so I pulled out my cellphone and dialed. A guy named Harv answered and as luck would have it he was the landowner. I explained who I was and asked if I could take a look around for fossils. He was friendly, but advised it was probably too late in the day to hike to the prime collecting areas on the eroding slopes of the mesa. He suggested I go back to where those big targets stood off the road. One marked a spot that visiting researchers had excavated, I might have some luck there.
I thanked Harv and doubled back. I spotted the target in question perched atop a little hill a couple hundred yards off the road. If I’d known anything about geology that lonely limestone pimple rising out of the otherwise flat plain might have given me pause. As it was, I had about an hour of daylight left and was in a rush to find something before dark. I pulled the car off on a solid shoulder and emptied a little Playmate cooler to use as a bucket, then strode through the sagebrush toward the hill. Fortunately, there was no snow at this elevation. My eyes scanned the bare ground beneath the brush, it was packed and gritty, with small stones “floating” on top. In this way I chanced upon a piece of baculite: smooth, oval in cross section, florid sutures showing. A good omen. I hurried on to the hill.
With the sun sinking behind the mountains I scrambled all over that craggy pile inspecting chunks of mustard colored limestone for signs of ancient life. And I began to see them: lenslike shells and fusilli fringed arcs peeking out of the rock. I didn’t have time to start splitting stones, so I just filled the cooler with anything that looked promising. As the sky turned purple Pueblo became a glittering oasis of streetlamps in the distance. I lugged my overflowing cooler back to the car.
Baculite Mesa in the background, partially excavated teepee butte in the foreground.
Back on I-25 I raced north toward Denver in high speed holiday traffic. I barely had time to notice the brake lights of the SUV ahead of me flash to veer onto the center median and avoid joining a multi-car pile-up. I heard the screech of brakes and the crunch of fenders behind me as weeds thrashed my undercarriage and I prayed not to snap an axle. As I shot past I stole a glimpse of the cause of this mayhem: someone had collided with a big antlered buck in the middle of the highway. I was luckier, arriving at my brother’s an hour later, shaken but intact.
Thanksgiving Day was balmy, in the 60s Fahrenheit. While the turkey roasted I sat on the back porch with my niece and nephews splitting rocks. They revealed an abundance of small clams: Nymphalucina occidentalis. Also embedded in the limestone were fragments of ribbed ammonites, or rather, the shells of those remote ancestors of squid and octopus. One auspicious hammer blow revealed the catch of the day: a three inch coil of some free-form heteromorphic genus, possibly a Didymocerus. Pretty good for an hour of swiftly improvised collecting.
Ammonite fragment (photo courtesy M.A. Ryan) Cretaceous clam
Harv had said something about a thermal vent when he directed me to that little hill. Some legwork on the internet revealed the full story. The site was a teepee butte, formed around a methane seep atop a fault line in the floor of the old Western Interior Seaway. That stretch of Colorado is famous for them. The buttes enclose petrified “extreme environments”. The clams and ammonites I collected are indicative of this highly localized ecosystem, different from that of the Pierre Shale of Baculite Mesa, though both date from 76 million years ago. These buttes may offer clues to the end-Cretaceous or other extinction events, if not a literally smoking gun. They are also of keen interest to space agency exobiologists who hope study of their “extreme” microbe fossils will help identify similar forms in extraterrestrial rocks. ET meets the K/T. Cool.
The next day Bill, inspired by my adventures, piled us into our cars and led us 15 minutes across Denver to Dinosaur Ridge. We pulled onto the shoulder of West Alameda Parkway and braved an arctic gale to gawk at giant hadrosaur footprints embedded in the steep rock face. Just another of seemingly innumerable paleontological attractions sprinkled across Colorado. I had barely grazed the surface. Alas, like the dinosaurs whose tracks vanished away up the ridge, I was just passing through.