Geology 101: The Shoofly Oolite


WC knows geology isn’t the most popular topic around Wickersham’s Conscience, but today, at least, it can take your mind off of taxes.

WC has written earlier about the massive Lake Idaho that filled much of the western Snake River Plain, in southwestern Idaho. Lake Idaho was an extremely long-lasting freshwater lake, possibly as long as 10 million years. Over that kind of interval, the lake repeatedly deepened and then became shallow; absorbed massive amounts of airborne and waterborne volcanic ash, and went through multiple changes in its water chemistry.

All those variations created a complex geologic history, and one place a part of that complexity is the Shoofly Oolite. WC and Mrs. WC visited the Shoofly Oolite yesterday.

The name “Shoofly” — a creek, road and rock formation — seems unknown. But oolites are geology, grains of sand encased in multiple layers of calcium carbonate. They are a shoreline artifact, usually when shorelines are expanding, where the oolites are tossed up on the lake’s beach, and then washed back in to the water. With the right water chemistry, high in dissolved calcium ions, the grains of sand become coated with layer upon layer of calcium carbonate. After a time, they look something like this.

Shoofly Oolites, about 5 times life size; 100mm f2.8 at 6 inches

Shoofly Oolites, about 5 times life size; 100mm f2.8 at 6 inches

The oolites are the roundish, whitish pebbles, mixed with coarser volcanic and sedimentary fragments. The oolites became buried in later lake sediments, and welded together into a oolith, a form of rock consisting of these small oolites. While a little tougher than siltstone and mudstone, it will pretty readily decompose.

Decomposing Oolith cobble, about 12 inches across, Shoofly Oolite formation.

Decomposing Oolith cobble, about 12 inches across, Shoofly Oolite formation.

It looks like a cobble covered in sand. But the sand is from the decomposed cobble. Weathering — water, ice, wind and sun — reduce ooliths to a coarse sand, consisting mostly of oolites. The larger lumps are shells from the fewah water mollusks that inhabited Lake Idaho.

Fossil mollusk shell (possibly a snail?), Shhofly Oolites, Idaho

Fossil mollusk shell (possibly a snail?), Shoofly Oolites, Idaho

The semi-hard oolith erodes under the Idaho weather into amazing shapes. Only the lake beach never looked so strange and bizarre.

Weathered oolith, Shoofly Oolite formation, Idaho

Weathered oolith, Shoofly Oolite formation, Idaho

Wind and water erode the oolith differentially, creating the 200-foot high domes shown here. Lower down, there are 50-foot high cliffs.

Oolith Bluff, Shoofly Oolite formation, Idaho

Oolith Bluff, Shoofly Oolite formation, Idaho

At the base of the oolith, you can see the layers of lake bottom from periods when conditions weren’t right for oolite formation. The black and light gray bands demonstrate the changes along the lake shore. Lots of organic materials, like a swamp, will color the sediment dark; fresher water, low in sediment creates the lighter colors.

Fossil lake bed layers, Shoofly Oolite Formation, Idaho

Fossil lake bed layers, Shoofly Oolite Formation, Idaho

(Mrs. WC and Tali the Geology Pup, included for scale.)

It may be all desert and sagebrush now, but the geology says that for a very long time it was a lake. The evidence is unmistakable, and worth a visit.

One cautionary note: when it rains, and it drizzled throughout our visit, walking on the oolites is much like walking on well-lubricated bearings. When they’re on a layer of lithified ash, or basalt, it’s kind of slickery.

So let’s hear it for Lake Idaho. Our visit to its shores was a lot of fun.

 

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