This past October I drove down to the San Rafael Swell with my two sons and we did a couple hours of exploring/hiking along and near the Moore cutoff road. We stopped at the pictograph boulders near the road just after the road descends through Molen Reef. A few miles beyond this there is a less developed road heading north which takes you to Short Canyon. Here we saw some very interesting geology.
There is a layer of the Mancos shale here where you will find huge spherical boulders. These are concretion spheres that apparently formed by chemical accretion in a warm shallow sea. Many of these boulders are 15 feet or more in diameter. They can be seen very clearly along the north wall of the canyon, but they can also be seen in the south wall if you look closely. As you drive or walk further up the canyon the elevation of the canyon floor rises and the boulders eventually get close enough that you can easily climb up and examime them.
This is a geological oddity that is well worth a side trip if you are in the area.
Here is a shot of the layer of shale with the boulders
A telephoto view of some of the boulders; these are 10 - 15 feet in diameter
A close-up of a sphere further up the canyon
Another sphere that has extensive weathering - notice how it erodes in layers like an onion
Alan strikes a heroic pose. This photo is mostly to give a sense of scale.
Here is the location of Short Canyon in Google Maps.
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