Fast forward 2 ½ weeks. As I traditionally do in Spring and Fall, this past weekend I headed down to Gooseberry Mesa with friends. The timing and the group was a little different; OCRick, Clean Colin, Vicente & Coryalis went down in March, when I was unable to join. This was the make-up trip, attended by me, KanyonKris, SkiBikeJunkie and Hunky Neighbor, joined Saturday night and Sunday on Little Creek by Special Guest Star Coryalis and his Wife-2.0-the-Excellent-Upgrade*.
*A long pseudonym, but totally true. We all love her.
It was an excellent group, in that we were well-matched in riding preferences, ability and endurance, and also in that we are all Excellent Campers. The relatively late date meant plenty of daylight, enabling us to pack 4 great rides into 2 days and 2 evenings.
Tangent: I must say that SBJ, who- astoundingly- had never ridden anywhere in the St. George/Hurricane area before this trip, seriously raised the bar on the culinary aspect of the trip. Here he is in action, preparing a delicious meal of steelhead salmon, baked in foil over campfire coals. He put my rather simpleton-breakfast-burritos somewhat to shame, though my fireside martinis seemed to at least partially redeem me in the eyes of my campmates.
On Saturday we rode Gooseberry Mesa, which is always a great time. Here’s a snippet from the Hidden Canyon section, in the interior of the mesa, following Hunky Neighbor:
Gooseberry Mesa is a blast because the slickrock outcrops provide all sorts of ramps, chutes, sidehills and gullies on a high-traction surface. The rock is clearly different than the “traditional” slickrock of the Navajo formation that you ride around Moab, on the Slickrock or Porcupine Rim trails, for example. The Gooseberry slickrock, while (at least) as high-traction as the Navajo, is much grayer/whiter, generally a bit rougher, and interestingly, often has small pebbles- up to 2” in diameter- embedded in it. Its geologic name is the Shinarump Conglomerate. Here’s another fun section of Hidden Canyon, and if you look at the gully sidewalls at about 0:29 – 0:33, you can make out bands of embedded pebbles about mid-way up the gully walls on either side.
All About the Shinarump
All around the greater St. George area, and in fact across much of Southwest Utah, Northern Arizona, Southern Nevada and (I hear) Northwest New Mexico, you’ll see mesas capped by Shinarump. This is because it’s harder and more erosion-resistant that the layers immediately below it- which we’ll come back to in just a moment. The Shinarump marks a transition from the Moenkopi formation, laid down around 240 million years ago, and the Chinle formation, deposited around 200 million years ago.
Extra Detail: The Navajo formation is more recent (maybe ~175 million years ago), and therefore “above” than the Chinle, so you won’t see it on Gooseberry Mesa, but you will see it up the road (and higher up) at Zion. To my knowledge there’s no Navajo-slickrock mtn biking in the St. George/Hurricane area. In between the Chinle and the Navajo are the Wingate and Kayenta formations, the former of which produces some of the spectacular cliffs by and above the White Rim trail in Canyonlands, as well as the deep side-canyons of the lower Dirty Devil*.
*Specifically Twin Corral Box, Sam’s Mesa Box, and (the middle stretch of) Happy Canyons. The Robber’s Roost complex is Navajo.
A couple layers above the Navajo is the Entrada formation, laid down ~160 million years ago, and consisting of smooth, fine-grained, easily-weathered sandstone. Both St. George (Church Rocks) and the Moab area (Bartlett Wash) have Entrada-slickrock mtn biking, which is often the smoothest slickrock around, though not as high-traction as either Navajo or Shinarump*. (The arches of Arches National Park are Entrada sandstone.)
*Especially the occasional- and super-smooth- whitish bands of Entrada, which can be tempting but treacherous. The darker Entrada is higher traction. Maybe the iron oxide in the rock binds the grains together?
I should be careful when I say “above”, BTW. What I should say is “deposited on top of.” Subsequent crustal faulting has raised, lowered and tilted layers all over Southern Utah. This is why you can also ride Shinarump on Zen trail down in St. George, 2,000 feet below Gooseberry, and why the Entrada sandstone of Church Rocks is 2,200 feet (!) lower than the Gooseberry Shinarump as well- because the Hurricane Fault* has raised everything East of La Verkin/Hurricane. And other local, smaller upheavals have also mixed things up; Nearby Slickrock Swamp Trail North of Rockville is also Shinarump, and 1,200 feet below Gooseberry, despite being on the same side of the Hurricane Fault.
*Which I explained in this post. Man, it is like I have a post for everything.
I’ve read 2 possible explanations of the formation of the Shinarump. The first is that it’s the result of a shallow, receding sea, and that the pebble bands are the results of beach/shallows deposits. The second is that it was laid down by the sediment deposits of braided streams, which explains the bands of pebbles well enough, but seems to me hard to swallow given the huge area covered by the member, and the relative uniformity of it. The Shinarump is almost always between 40 and 100 feet thick, and is present over a good chunk of 4 states. How could braided streams cover such an area for so long?
Interestingly, nearby Little Creek Mountain, also capped by Shinarump, seems to show pebble bands much less frequently. Many of the Shinarump areas on Little Creek are pink-ish in hue- something you almost never see on Gooseberry- due to the presence of iron oxide in the stone.
I mentioned that the layer just below the Shinarump is the Moenkopi formation, easily eroded, and not high/sheer-cliff-forming. You see it frequently from the Mesa rim, forming beautiful red and what bands on the slopes below. Geologists divide it into 6 alternating layers, 3 red, and 3 grayish-white, but there are varying sub-bands within the layers, so it’s not always easy to pick them out. The topmost layer though is easy to pick out. It’s a red layer, called the Upper Red Member. It’s usually about 250 – 300 feet thick, and if you pay attention you’ll notice that the rock/soil immediately below the Shinarump is almost always deep red.
You can get good glimpses of the Moenkopi color-banding in the background of this clip, at 0:03, 0:09 and again at 0:19.
Tangent: I had my helmet-cam rolling at the time, so here’s what it looked like from my POV.
Nested Tangent: I was the only one of our group- and the several other groups hanging out by the point at the time- who rode the little point loop. The exposure there never bothers me, but, oddly (to me, anyway) it seems to make most other folks uneasy. After I rode it, SBJ claimed he “couldn’t even watch.” I thought he was joking, but check him out at 0:25 – 0:28, standing by is bike on the left in a pink Fat Cyclist jersey. He really is looking away! (Then again, so is Hunky Neighbor. Maybe they’re just pretending they’re not with me…)
You never ride on Moenkopi on the mesas, but that’s exactly what you ride on down on the bench level, on say JEM or Gould Rim trails. Here’s a cool clip of descending upper JEM*. At the start, I’m riding on soil of the Upper Red Member (#6 out of 6, going from bottom to top.) At 0:21, you’ll see the soil change suddenly to white as I transition down into the Shnabkaib Member (#5 of 6) consisting of friable shale with plenty of gypsum. The Shnabkaib is usually about 270-300 feet thick, and although I didn’t have an altimeter, this seems to be ballpark-roughly how much I descend before crossing the thin capstone layer at 1:39 and transitioning over the next 20 second down into the Middle Red Member (#4 or 6).
Seriously, how cool was that?
Another interesting thing about Shinarump slickrock- and something you see frequently on both Gooseberry and Little Creek, as well as nearby Guacamole- is petrified wood. It’s all over the place, and I’ve read (again!) 2 different explanations for its presence. The first is that its petrified driftwood deposited when the conglomerate was deposited, either as shallow sea or braided streams. But the alternative is that it’s from a higher, now-eroded layer. The Chinle formation consists of 4 layers above the Shinarump, the 2nd (from the bottom) is known as the Petrified Forest Member, consisting of shales, limestone and volcanic ash, and containing numerous deposits of… wait for it… petrified wood*. The wood is harder and more erosion-resistant than the surrounding soil, which could have left it deposited on subsequently-exposed Shinarump.
*This same Petrified Forest member is what you’re looking at when you visit Petrified Forest National Park in Arizona.
I’m not aware of any spot where one rides through the Petrified Forest member on Gooseberry, but you do ride through the layer below it in spots. On the mesa, one of the faster and more dramatic sections is moving East along the South Rim. In a couple of spots the trail actually moves up onto a layer of beige-ish-white clay/soil overlying the Shinarump, which is the Lower Sandstone Member (#1 of the 4 layers, counting up), consisting mainly of fine quartz grains mixed with gypsum, iron and lime, and usually about 100 feet thick. In this clip you can see Hunky Neighbor climb up of the Shinarump and onto soils of the Chinle/Lower Sandstone at 1:08. At 1:35 – 1:40 he rolls back down onto the Shinarump, where we continue for several more seconds in the series of drops known as “Rattlesnake” before I violently (and audibly) flat.
Man, what a great weekend. Who would’ve thought geology could be so fun*?
*Well, Jube, obviously. But I mean a non-geologist… Hey Jube, how about some more geo-posts??
We headed home Sunday afternoon after revisiting Little Creek, and I quickly un-packed and re-packed before flying out for a week on the road the following morning. Looking forward to finally getting home this weekend, and catching up on the Spring that’s been flying by. AW called me here in Florida last night; the Lazulis are back.