Houselog Creek to Blue Mesa Reservoir: Two Tertiary calderas in one morning
Wednesday August 9, 2017, 70 miles (113 km) – Total so far: 135 miles (217 km)
Last night, after a nap and a feed and the passing of the afternoon storms, I climbed the adjacent hill for a look around. It was a steep climb of several hundred feet to where ignimbrite sheets (i.e. consolidated pyroclastic flows) capped a long ridge. Where we were camped is on the northeastern edge of the San Juan volcanic field. This volcanic field covered much of the southern Rocky Mountains in the middle Tertiary (about 35-25 million years ago). The field spewed out impressive volumes of volcanic rock – at least 22 major ash sheets (each 150-5,000 cubic kilometres of rock) originating from calderas (10-75 kms in size) after a period of flows from clusters of stratovolcanos.
I zig-zagged my way up the tuff and found a patch of shade underneath a large Ponderosa pine. I turned. And turned. And turned. And turned. I had 360 degree views. That little bit of elevation gain gave me more of a google earth view than I expected. I immediately felt whole and alive and spectacularly tiny. This is why I ride. I am not a people person – unlike most touring cyclists, I don’t tour to meet people and that is never the highlight of a ride for me. I tour for landscapes, particularly mountainous ones with interesting or complex geology. It is this feeling – the feeling of being so tiny in such a complex, old and changing landscape that makes my heart sing.
I stood up there for a long, long time, wandering around on that flat-topped volcanic ridge. I could pick out lava flows and plateaus and the paleochannel for Saguache Creek. I reveled in the tuff cliffs and the erosion of valleys between the many, many different sheets and flows. Lightning arced through dark skies far to the east. The long, low-angle golden planks of the setting sun licked the lava tongues that reach down into present-day Saguache Creek. I tried to trace the paths we will ride over the coming days through the various jumbles of rock spewed, flowed and laid down around 30 million years ago. Time seeped into my soul and pushed out all the problems of present-day. Rejuvenated, we returned to the tent to let sleep do the physical side of recovery.
Now, at 6.30 am, we are pushing the bike back to the gravel road and then twisting down between tight walls of Saguache Creek Tuff back to the main road. Low-angle sunlight strikes the high vertical walls but leaves us in the shade. It chases us down the main road as we gently climb up the wide valley. Today we are going to ride through two calderas – and the nerd in me is quite happy about that.
Our first view in the main valley is of the ridge on the opposite side of the valley. It has at least four different sheets of tuff from 28-33 million years ago. Right up next to the road in places on our left, we pass next to rhyolites and tuffs originating from various calderas. We also pass a section of Conejos volcanics which predate the calderas and were erupted from the stratovolcanoes.
The views to the left and right are complex and impressive, but right in front of us is a cause for concern. There are signs for HOT OIL and LOOSE GRAVEL for NEXT FIVE MILES. That is not good. But our timing is. We beat the roadworkers to their chipseal project. But we definitely agree that the road needs the work – the frost heave cracks are huge, jarring and frequent.
Just as this section ends we curve below an adjacent hill of Fish Canyon Tuff (from the La Garita caldera 28 mya), cross the creek and get a view of part of the east wall of the North Pass caldera rim. The character of the ride changes here. So far, from Saguache, we’ve been following the wide valley hemmed in by tuff cliffs and lava flows on the far walls. But here the road starts to climb and the topography grows more lumpy and chaotic.
Here we are riding into the old caldera on a climb up to some Carpenter Ridge Tuff (from the Bachelor caldera 27.5 mya). The North Pass caldera is really just a pile of rocks about 7-10 kms across and its rim is not visible in many places. It is filled with its own eruptions and with the flows from other, more recent calderas (like the Bachelor and La Garita calderas). So it’s not all that obvious at the surface what is going on. But it is still so much fun to know we are riding through our first caldera of the day.
We are also riding through the Sheep Creek fault zone – which adds to the various folds, drops and diversity in the landscape to our right. The fault zone is associated with the rifting in the San Luis Valley we left behind yesterday. That valley is splitting apart in a great rift and these are associated extension faults.
I delight in the lack of traffic this early and all of the volcanic rock just everywhere. We can even pick out the contact between two different tuffs. Ooooooh, I am such a nerd. And then the climb begins to North Cochetopa Pass. Traffic has picked up a bit – a car every 3 minutes or so – but it is still pretty light. The climb is four miles, but none of it is steep.
The road sticks to the valley wall and rises high above the creek. (There is a Forest Service road down below somewhere that follows the old stage route – an adventure for another day!). There is so much joy to be had in gawking at the road cuts on this climb. There are many different rock types through here, including four types of debris flow from an unknown source. Most exciting to me is columnar jointing in a post-caldera flow of dacite. It’s fist-sized and leans into the hill. I don’t think I’ve ever seen such small columns or columns present in anything but basalt. My day is made. We curve on up the climb – what an absolute joy!
My butt does not agree with my head. It is not happy that we have not been on the bike much this North American summer. But soon enough we reach the top of the pass. We are not in the groove physically yet, but at least that was another easy pass and our geology groove is definitely…. groovin’.
The downhill is fast and the pavement good on the steep bits. We are paralleling the margin, or edge, of the North Pass caldera here. We are also crossing the Colorado Trail and Continental Divide Trail. There are two long-distance hikers standing on the large switchback curve. They look up and one of the guys gives me a big thumbs-up. I just nod. At 40 mph, out of the saddle, on a curve, I cannot wave. I am just not that skilled.
(Apologies about lack of pictures for the next bit – I guess I was too caught up in all the nerdiness to take pics – the two I did take are blurry).
The road weaves through tight rocky walls and higher hills in the background. These are all Conejos rocks from the stratovolcanoes and a little bit of tuff from the Bonanza caldera (which predates the North Pass and Cochetopa Park calderas by a few million years). Soon enough the downhill run passes through the Cochetopa Park caldera rim and we find ourselves in a more open area with long views. Caldera number 2 for the day – how much more could you want?! Two Tertiary calderas in one day. That is just such good stuff.
The road climbs and falls on intracaldera fill of ash and tuff. There are tremendous views of Razor Creek Dome and Sawtooth Mountain on the northern rim of the caldera in our forward view. Off to the right are all sorts of fanglomerates (my favourite word for today – which means a whole bunch of volcanic rocks mixed together in an alluvial fan that has slid down the caldera margin) high above us. To the left is the resurgent Cochetopa Dome. It is a series of three rhyolite flows formed after the caldera collapsed. There are dykes down in the basin, too. We will again ride into this caldera and see the dome from the other side in a few days.
I stop for a snack break at the top of a hill where I can get a great view of the northern rim and the caldera outlet at Cochetopa Creek. I can gaze all around at this morphologically intact caldera. I just can’t get enough. Get your nerd on, today! I am just in awe of the magnificent morphology display, and I’m loving every second of this. My butt is not loving it and various other things hurt, too. But this is good, good stuff.
I say out loud, “see you again, soon” as we pass out of the caldera. We are now in for a visual treat as Cochetopa Creek cuts down through Precambrian rock in a multi-mile scenic cruise downhill through a tight canyon. This reminds me of Spearfish Canyon in South Dakota a little bit – just smaller and without the variation in rock type. We twist and turn and curve with the river through the tall, jagged walls.
But whew, the traffic is not fun. I’m getting bursts of 7-10 cars from behind (from that chipseal project way back on the other side of the pass) plus others, and cars coming from Hwy 50 every 1 minute or so. I would never call this “light”. It is prime tourist season, and I am convinced Colorado has officially become crowded. The whole trip will feel over-peopled – this is just the first taste.
We stop for drinks not far from Hwy 50 and then pedal down its huge shoulder into Gunnison. The traffic is constant but it is no worry since we’ve got fresh asphalt and a good distance between us and the vehicles. Gunnison is very busy. I use back roads to get to the Dollar Store to restock snacks. Then I head to the park to send updates/check in and get the weather forecast, since I won’t have Verizon service out at Blue Mesa Dam. Then it is over to stand in a long line at Subway to get a sandwich for dinner. I cannot believe how many people are just everywhere – the Colorado I once loved and lived in is long, long gone.
The wind has picked up from the west – pretty much a predictable thing in the afternoons on Hwy 50 from Salida to Montrose. I piss off all the protesting body parts and pedal across the Gunnison River flats to the west. I don’t much care for the tent sites or squishiness of the KOA in town, so we’re heading out to Stevens Creek campground for the night. It’s not so great either, but its sites are not squishy. Most importantly, it has water and a toilet – I don’t know when we will next have access to either of those, so we’ll camp there for convenience rather than anything else.
The road passes many put-in/take-out points and parks along the river before weaving into the Gunnison River Canyon. The wind funnels through here and I’m pushing against 20mph winds. Grunt. Grunt. Grunt. Shut up, body, we looooove this!
Stevens Creek campground is all in the sun and sagebrush right along Highway 50. Loop C is furthest from the road but the most crowded. I set up in Loop B where I will not have to listen to 10 RV generators all evening. The only others in Loop B are a Japanese family ‘camping’ in a van and hilariously hunting for firewood anywhere and everywhere, an old guy in an RV who smiles and waves, and a group of three roadworker guys off in the distance who are very happy when I strip down to bra and undies to have a bucket bath. I have no shame. I will never see them again, and I am here for the water and the possibilities it provides.
I take the guys down to relax at the dock in the evening. It has been another fantastic day. It is also the first day in three weeks where it hasn’t stormed for most of the afternoon. Tomorrow looks decent, too, which is good, because tomorrow we head onto gravel. I don’t worry about it too much, since the dirt roads here don’t have much sticky clay and are generally quite rideable except for maybe during the storm itself. But, still, I prefer a quicker, less gluggy surface and no lightning danger when possible. We’ll see what the day brings tomorrow. Good night – time to let the achy bits recover once again.