Monday, April 23, 2018

Mystery Sandstone Spheres Follow Up (Part 2)

This is a continuation of my immediately preceding post.

Where the mantle is absent, such as bare spots on ridge tops and gullies, especially on their sloping slides, the spheres commonly cover much of the exposed surface. The above view is of the side of deeply incised canyon with abundant exposed spheres, most of which are about 10 mm in diameter. Staff is 2 m in length.

Close-up of spheres from previous photo. Geological Society of America scale, 10 cm intervals on left side of scale.

Freshly broken layer shows no spheres on fresh surface (along right side slab) compared to the older surface with many spheres attached. This photo is very important because it shows that the  spheres form on exposed surfaces and are apparently the result of weathering (i.e., formed after the sediments became rock).

Spheres of various sizes on vertical surface. Within a given layer, the spheres are fairly uniform in size. In incised washes, the spheres occur in multiple layers, which can be interspersed and/or bounded by sphere-free layers. The beds that contain them can be over a meter in thickness and can be completely covered by the spheres on exposed surfaces.


The spheres are not armored-mud balls (interior is mud and the exterior is coated by an assortment of angular particles of many sizes), which form by rolling along the floor of a desert stream. Armored-mud balls do occur in the same area as the spheres but are comparatively uncommon; one is shown on the right. On the left, is an unusually large and liberated sand sphere with its characteristic smooth surface.


The spheres are not concretions. Unlike concretions, the spheres have a uniform composition all the way through (see the original post), without concentric shells, and they do not leave concave depressions behind where they were formed. Concretions are hard solid masses that form slowly via chemical changes induced by groundwater percolating through the sediments before they become a sedimentary rock.

The presence of spheres in a road cut along Interstate 15 (see index map in preceding post) indicates that the spheres do not require geological time to form. Interstate 15 was completed in 1964, so the road cut containing the spheres was made sometime just prior to that construction phase. Because the spheres require surface exposure to form, this puts an upper limit on the formation time at the road cut to less than 55 or 60 years. They might well require a much shorter time to form than that.  

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