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  • Amanda

Bikes and Rocks

Updated: Nov 10, 2020

New miniseries..."Bikes and Rocks". It's a geologic record of a backroads bike trip we took this summer across a respectable chunk of the Southern Rockies, from Trinidad to Gunnison. Every time I passed a good rock outcrop, I parked the bike and took a picture, any excuse to take a break. Plus a way to have a little fun with geology (I know those two words don't go together in most people's minds). But let's give it a try. Here is an outcrop--or maybe it's a huge boulder--of leucocratic gneiss in the Sangre De Cristo range. We're on the backside of Great Sand Dunes National Park, coming west on Medano Pass. A close look shows it's a hot mess of metamorphic rock, full of kinks, swirls, and hints of layers. Unbelievably ancient volcanic ash and lava that had since been buried, folded, faulted, and pressure cooked, long before the Sangres rose up. They just exposed what had been going on way back when--back when oxygen was just beginning to appear on Earth. My mind is blown, and we hadn't even hit the sand dunes yet.

Part 2. "Bikes and Rocks". Slowly moving west on our summer's bike trip, one pedal at a time. Made it through the Sangre de Cristo range and ran straight into something that shouldn't have surprised us...sand. After all, we were in Great Sand Dunes National Park. It's a geologic phenomenon that's easy to gloss over if you're more interested in intact rocks, but hard to ignore when your bike mires down after 4 pedal turns... over and over again. It turned into a painful, slogging, 4 mile push that humbled us. But this patch of sand in the high and dry San Luis Valley is rock broken down AND rock in the making. It's tiny pieces of quartz and volcanic rocks from the San Juan Mountains, just upwind. They blew up off the valley floor, funneled toward the Sangre's low Mosca and Medano passes, got deflected by Blanca Peak, and built up here, on the eastern edge of the Rio Grande Rift. It's still dropping, and filled with over 10,000 feet of sediments. Once things get that deep, pressure and heat start making rock out of the loose grains. n.b.. Getting through it was so consuming, I forgot to take a picture until the very edge, where vegetation was starting to get a roothold.

Part 3. A great place to lean my brave little bike. A few miles east of Carnero Pass we came across a reminder of the volcanic activity that made this part of the southern Rockies what it is today. Now it's a rolling, forested highland, with gentle drainages and pretty meadows. It used to be a molten hell-scape, back in the Miocene (only 5-23 million years ago), smothered with ash, lava flows, and concocting up other igneous rock structures like stocks, plugs, sills, laccoliths, and dikes. Take this one: a dike which looks like it's made of basalt, and must have formed back when this valley was some kind of country rock pushed around by the burgeoning magma underneath. Cracks happened, then lava squeezed into them. The country rock (not a music genre) eroded away, leaving this black section of wall to tower above the land around it. A big clue to how things used to be around here.

Part 4 Bikes and Rocks, or what I did this summer. Now that snow is here, I'm thinking how nice those 6 long hot days on the bike were. A slow toil through Colorado's mountain geology, one pedal at a time, and along just one narrow transect through the mountains. Nevertheless we ran into a lot of geology, some of it glamorous, some not. Take this old landslide deposit east of Cochetopa Pass. Not much gets said about landslide deposits, they don't have glittering crystals or cool fossils as a rule unless they happen to pick up those kinds of rocks as they tumble down a slope. But what landslide deposits do contain is a lesson about how rocks work--the breaking down part of the rock cycle: the part that's about rocks weathering, fracturing, falling, and getting ground down to bits. Here in this deposit, they're just a little way along their journey toward getting buried, (and deep) where the pieces will ultimately be compressed and form new rock all over again. Surprised there's a rock cycle? I was. In retrospect, it makes bicycling the perfect way to appreciate geology. A painful pun, I know.

Part 5. Moving onward, still in volcanic country in southern Colorado--Cochetopa Hills-- an area of the state that looks just like it did 50 years ago. Another cool roadcut, this time into what seems to be the dacite of East Creek Pass. It's tan, fine-grained, hard and glassy at a micro-level, and breaks into blocks. Back when it was flowing in a tumult of superheated pumice, ash, and gas that rocketed across the landscape, it was almost liquid and gritty. Evidently, it came exploding out of some volcanic vent and accumulated here in the recently collapsed North Pass Caldera, nearly filling it. This giant caldera was so deeply buried, geologists discovered it only recently. The ash was over 1,000 degrees F, so hot that it all melted and fused together once it came to rest. Just one particular type of rock among many spewed out in an explosive burp of magma. This batch happening to have a lot of silica, and this location happening to be situated far enough, but not too far from the vent to retain the right amount of heat and ash of the right particle size to fuse together just so to make dacite. That was then, some 32 million years ago. This is now, and because it's a hard, thick layer, it still dominates much of this landscape. It shows up in the cap that tops mesas and buttes for miles around in the Cochetopa Hills. Now what are the chances of that?

Part 6. Summing it up. Over the last pass and speeding down a paved highway toward Gunnison. Mind more focused on getting there than taking in the scenery. And with that, no mental jumping off points for wondering about what I'm seeing, what it is, and how it came to be there. Mental distractions are crucial for long distance biking. The layered sandstones near Trinidad, the volcanic structures around the Spanish Peaks with all their radiating dikes, the metamorphic rock of the high Sangre de Cristo range, the sand of the San Luis Valley--these were some of the big distractions for the first half of the trip. Not just pretty scenery, but also a landscape puzzle to figure out. Harder rock, softer rock, flat rock, tilted rock--they created the terrain that made the biking hard, or made it easy. Then the huge area of volcanics west of the San Luis Valley which built the La Garita Mountains and the Cochetopa Hills. Here was a different landscape, more gentle, and fertile from volcanic rock. And with each formation we passed, I knew there was a story behind it once I could do a little online research, a story that would provide a glimpse of ancient Earth, hidden secrets, and profound processes.

Too tired to stop unless absolutely necessary. This outcrop of metamorphic rock met the criteria. Dark, hard, and muscular, glistening in the sun, this was something different. I could tell it wasn't volcanic, or at least not in its latest incarnation. It turns out there's a good chance it's Precambrian metabasalt, originated as lava flows long before being squished and recrystallized as this massive, blocky rock with a sexy sheen. It's so old it metamorphosed before there was multicellular life, no telling when that original lava flowed. But now it's joined the ranks of other Precambrian basement rocks that block faulted here in Colorado into a series of long north-south lying pieces, some raised up to form the high Rockies, others dropped down to create the big parks. And so it was, on this last day of the ride, we found the very essence of the Southern Rockies in this outcrop: the Precambrian core.

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