S t o r i e s i n S t o n e : T h e R o c k s T h a t B u i l t D e n v e r J u l y 1 5 , 2 0 2 3 R o c k y M o u n t a i n A s s o c i a t i o n o f G e o l o g i s t s

T a k e a w a l k i n g t o u r w i t h R M A G & l e a r n a b o u t D e n v e r ' s i c o n i c s t o n e b u i l d i n g s a n d t h e g e o l o g y o f t h e q u a r r i e s w h e r e t h e s e u n i q u e r o c k s w e r e e x t r a c t e d .

» CONTINUED FROM PAGE 22 enhanced the vagaries of slight indentations to form a close approximation of a clear print for photography. Today, using mobile phones the size of a pocket calculator, we can gather digital images that can be reconstructed into 3-dimensional models, rendered as color-coded elevation (or “heat”) maps, and even used to make perfect 3D prints from resin and polymers (Figure 7). These advancements replace the potentially destructive methods of yesteryear, which required tedious brushing of thin layers of liquid latex onto the track surfaces, waiting for hours as it cured, and then encasing the whole in a plastic mother mold to retain its shape after the latex had been pulled out. Now, regardless of lighting conditions, climate, or time limitations, data can be painlessly gathered in a non-destructive way in seconds instead of hours, days, or even weeks. Boosted by these game-changing technological tools, our work continues with new efforts focused on digitally capturing all of the exposed surface tracks and as many of the tracks preserved in cross-section as possible before these snapshots of the Paleocene erode, unceremoniously crumbling down the hillside and taking their secrets of the past with them.

Compiling all of our observations, measurements, maps, and comparisons to modern traces and trace makers, we have been able to come to some revealing conclusions. Approximately 59 million years ago, tectonic subsidence rates were extremely high in south-central Wyoming. Rivers delivered huge volumes of sediment from the surrounding uplifts, but these deposits could not keep up with the sinking, resulting in the nearby Western Interior Seaway flooding back into the area that is now the modern Hanna and Carbon basins as it had in the Cretaceous (Wroblewski and Steel, 2022). The newly flooded wetlands served as novel habitats for the local aquatic and terrestrial animals to colonize. Freshwater or brackish-water insects could live alongside marine anemones in bayhead deltas and distributary channels in what we call “the Goldilocks zone”, meaning it’s not too salty for freshwater organisms and not too fresh to eliminate all marine creatures. This was the setting in which the large mammals and probing birds left their marks.

In the distance, trees line a shoreline that marks the spot where forested, swampy land meets brackish water bay. Further off, the hazy purple outline of a low range of hills and highlands is barely discernable, though these uplifts are the source of the small streams and rivers that flow into the coastal lagoon below. Crocodiles glide through the freshwater and brackish channels, avoiding the salty waters of the lagoon. The constant rumble of breakers crashing against a barrier island to the east is like the roar of an oncoming train, but the calm of the lagoon’s water is only broken rarely by a breeze riffling across its surface or the rare storm that churns it into a roiling, grey-green cauldron. Wading birds resembling modern willets and sandhill cranes stride across the expanding tidal flats, displaying to each other, calling, and probing for invertebrates as swarms of biting insects rise up with a high-pitched whine every time the breeze dies down. The falling tide exposes anemones to the air, spurring them to retract squirming tentacles back into their barrel-shaped bodies and retreat into the silt to patiently wait for the next high tide. Distinct sounds of heavy feet splashing, squishing, and sucking out of the sand and mud are punctuated by snorts, rumbling growls, and the occasional sneeze. A group of cow-sized mammals, seemingly a cross between river hippos and brown bears, is schnuffling for edible debris washed up by the previous night’s storm. Adults and some half-grown juveniles, born earlier in the spring and now enjoying their first summer trip to the seaside, traverse a path that they will use for years to come. They crave the salt and other essential minerals that the plants they normally browse upon do not provide. Some wade into water deep enough to cover all but the tops of their heads to escape the relentless heat and blood-sucking insects. A bear-like mammal, the same size as the largest of the hippo imposters, lumbers along the tidal flat, stopping to sniff at bits of flotsam that have washed up with the last high tide. These denizens of the coast have no way of knowing that their activities will be preserved for the next 59 million years in the sediment below their massive, clawed and hoofed feet. They can’t conceive of our human-saturated future and will never know a world other than this coastal paradise. When their lives end, their bodies will be completely returned to the earth, leaving no evidence of their existence. It will be the descendants of the small, squirrel-like

» CONTINUED ON PAGE 26 primates chittering from the treetops on the shoreline that will one day stumble across their ancient footprints and walking paths and bring these giants back to life, if only as a daydream on this high plains landscape. If you learn to decipher the various trace fossils preserved in sedimentary rocks throughout the world, you too will be able to visit lost worlds that are at once alien and also eerily familiar.

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