Utah Rocks! - Episode 1: Goblins and Jasper Geodes

So we took a family expedition to Mars this weekend :)

Goblin Valley, Utah seriously might as well have been Mars. I have never in my life been in such an alien environment. There are people in the center of the photograph above - if you didn't see them at first, find them so you get a sense of scale. The mushroom-esque rock formations are called "goblins" locally, although this type of erosion feature is generally called a hoodoo.

Wandering around, between, under and on top of these expressive rock bodies, it was no wonder to me at all why the first white settlers to "discover" the valley called them goblins.

Getting there

Goblin Valley is easily accessible by car with a $15/car fee required to enter the state park, which is open year round. The surrounding area is mostly flat red dirt sagebrush desert, punctuated by impressive buttes with beautiful contrasting layers of red, white and brown sandstone.

Wild Horse Butte dominates the approach to Goblin Valley - shot through the passenger's side window (right side of the car for you backwards drivers out there) on the road in to the park.

How the heck?

It is tough to imagine how a valley full of thousands of free standing rock mushrooms comes to be. Allow me to explain :)

Wild Horse Butte

holds the key...

The four named rock formations which can be found in Goblin Valley can all be seen exposed on Wild Horse Butte: the Morrison Formation; Summerville Formation; Curtis Formation; and Entrada Sandstone. The rocks here date back to the mid to late Jurassic period.

Entrada Sandstone

The goblins only occur in the Entrada Sandstone layer, which is actually intermixed sandstone, shale and siltstone originally deposited in an inter-tidal zone when this area was on the edge of a shallow sea. The red color is staining from the presence of trace amounts of iron ore (hematite).

Intersecting fractures in the Entrada Sandstone beds create joints and places of weakness in the rock. Points of rock exposed at intersections of joints weather quicker than the surrounding rock because they have more surface area exposed to erosive forces, which eventually leads to a spherical shape (goblin/hoodoo) as the corners are rounded off (spheroidal weathering).

The characteristic mushroom shapes result where weaker layers of shale and siltstone underlay the sandstone body of a goblin; these weaker layers erode quicker than the sandstone goblin body, giving the appearance of a stalk underneath a mushroom cap.

Below you can see a group of four new goblins forming from a larger body of sandstone, but they have not yet weathered far enough to become freestanding bodies. You can clearly see the parallel cracks or joints that separate the sandstone into four separate bodies, and how the softer shale/siltstone layers underneath the sandstone are weathering quicker:

Curtis Formation

The upper boundary of the red Entrada Sandstone is clearly marked by the contrast with the striking light green-gray limestone of the Curtis Formation. The Curtis Formation limestones were deposited when the area was inundated by the Curtis Sea, and the color derives from the presence of the mineral glauconite.

Before you can fall, you must rise up

Goblin Valley is in the middle of the San Rafael Swell, a large area where older rocks from the Jurassic period were uplifted ~40-60 million years ago during the Pliocine. This uplifted area, or swell, raised the Entrada Sandstone up high enough to be revealed by erosion over the last 10 million years.

If the uplift event that caused the San Rafael Swell had not occurred, we would not have this magical valley of goblins today. The Entrada Sandstone formation occurs across most of the Colorado Plateau, but generally is covered by hundreds of feet of later sedimentary deposits. Only in areas where the rock has been uplifted (such as the San Rafael Swell), or in areas where a river has formed a deep canyon, is the Entrada Sandstone exposed.

Disc Golf with Jasper Geodes

If you have followed my blog for a while you may know that I am an avid disc golfer. Imagine my delight when I found out there was a disc golf course around the campground in the park, just outside of Goblin Valley itself. I can't say I have ever had a better view while playing disc golf before!

@yeti-the-dog approves

You will be walking through a delicate desert ecosystem while playing the disc golf course, so be careful not to step on low-lying cactus!

Vivid yellow and red jasper chunks litter the surface of the red dirt in the area of the disc golf course, washed out of the limestone in the surrounding buttes and exposed in the small gullies and washes that carry the occasional rain water across the red dirt ground:

I found some large jasper geodes with an interesting "bubbly" jasper rind surrounding quartz and other minerals. My later research tells me I should not have picked up these rocks from within the park boundary, but you can stop along Highway 24 outside of the park on your way in or out and collect jasper geodes legally. This website describes the location of the two major jasper collection areas that are along the road, as well as giving some information on their mineral composition and formation.

The geodes in this area were originally formed from nodules of anhydrate, an evaporite mineral formed from evaporating sea water, which later were covered in sediment and eventually dissolved when water again covered them, leaving empty pockets that the geodes formed in.

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All images are my own property. In addition to sources noted in-line, I used the following as a primary source:

The Geology of Goblin Valley State Park, Mark Milligan - Public Information Series 65, US Geological Survey

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