Behind the San Rafael Reef – the answer to geo-challenge 2.
Wild Horse Canyon – one of those beautiful Utah slot canyons – has been calling me for years. On a Friday in mid-October I was nearby with time to visit. “Great!” I thought ... naively. At the trailhead two of the three parking lots were full, and people were in line to use the restrooms. This is not unusual I later learned.
I opted for a road less traveled and drove to another place I’ve always wanted to visit – the San Rafael Reef, specifically the stretch just west of Utah Highway 24. The intricately carved pale rock extending for miles has always looked so inviting. But wait ... a reef in Utah ... a land of deserts and mountains ... far from any coast?
A reef in the desert. Photo by Jack Share of Written in Stone, used with permission.
Of course this isn’t a true reef. It isn’t even an ancient reef though this part of Utah was underwater in the past. It’s a ridge of fossilized sand dunes almost 50 miles long. Early travelers called these rock barriers “reefs” – Silver Reef, Buckeye Reef, White Reef, Capitol Reef, etc.
The reef of Capitol Reef National Park has domes reminiscent of capitol buildings. It’s part of the much longer Waterpocket Fold.
The San Rafael Reef was once part of an erg or dune field. Last week I mentioned the Entrada erg which used to cover much of Utah. This is the Navajo erg. It’s older – roughly 180 million years old (it’s hard to date as ergs typically lack fossils) – and was perhaps the largest erg on earth ever. Like the Entrada, it become sandstone after being buried under thousands of feet of sediment. Then during the Laramide orogeny – the mountain-building event that created the Rockies – the area was folded to create the San Rafael Swell (uplift) with the Navajo sandstone and other sedimentary strata steeply tilted on the east flank. Erosion has exposed and carved it into a nearly impenetrable wall ... a reef.
Steeply tilted flatirons of Navajo sandstone appear to plunge into the basin. Photo by Jack Share of Written in Stone, used with permission.
East half of San Rafael Swell. Sedimentary strata were folded quite steeply at the Reef; modified from source (2012).
Between Interstate 70 and the turnoff to Goblin Valley State Park, Highway 24 runs almost parallel to the Reef; they’re separated by less than three miles much of the way. A few dirt roads turn off west, and I found one that ended near a possible hike. It looked like a continuous route to the top, but I knew better.
Let’s hike to the top of the sand dune!
Hmmm ... which way now?
Like modern day dunes, these fossilized ones were hardly continuous for long. Slopes were dissected by gullies. It was up and down to connect walkable slopes, and you can’t just slip-slide down rock dunes like you can in sand. However we managed to wander to the top, taking in great sights along the way.Cross-bedding was everywhere, typical of dunes and wind deposition.
Near start of the hike (note car).
For awhile we followed cairns. Someone else had the same idea.
We found what looked like soft-sediment deformation. Perhaps wet sand slumped or moved under pressure. It certainly reminded me of soft-sediment deformation in the Navajo that I saw at Capitol Reef NP a few years ago.
Soft sediment deformation at Capitol Reef National Park (lower cliff).
In places, the sandstone had fractured along a network of joints.
Joints, with foot and dog for scale.
Plants thrived in gullies, and other nooks and crannies. Some managed to grow from small shallow cracks in the sandstone. Bonsai-like junipers and pinyons were common.
Pinyon-juniper "woodland".
Grass was thick in gullies that gather rainwater running off rock.
This Utah juniper is about two feet tall. It's alive and growing, don't be fooled by the dead wood.
Curl-leaf mountain mahogany.
Yucca in a crevice with a bit of sand. It's five inches across.
Hiking on the pale rock was a bit like being in a reflector oven – surprisingly warm. But shady spots provided relief for weary travelers.
Turns out we were hiking in the San Rafael Reef Wilderness Study Area.
Where on Google Earth; click on image to view details.
For more about the geology of the Swell and Reef, see the excellent and entertaining Geology of the San Rafael Swell by Jack Share, who kindly supplied several photos for this post. You will find more equally-spectacular ones if you follow the link.
