While taking a walk

Like any good Minnesotan, I appreciate a quality agate, and although I don't do a great deal of agate hunting, I *have* learned how to spot their characteristic luster. (I've never spotted anything big, but it's a neat trick.) When road work began this summer near where I live, I made it a point to observe the disturbed ground when I would take a walk. The glaciers that brought agates into the Twin Cities didn't linger where I am, but you never know. Until a couple of weeks ago, all I saw were chips, including the remains of one that may have been about the diameter of a dime had it not been shattered into shards by being run over (I didn't know they would splinter like that). This time, though, I noticed a red pebble with a cherty sheen on the path. I could tell it hadn't simply gotten paint on it, so I scooped it up to clean it for a closer look. Although it has microcrystalline quartz and red coloration, and certainly looks like it might be a Lake Superior agate from a few feet away, that's not what it turned out to be.

I'm cheating because I haven't yet showed a photo of one of the shorter sides, but this is actually a pebble of Mary Ellen jasper, a stromatolitic jasper that comes from the Biwabik Iron Formation. It's quite a bit older than any of the other fossils that have featured on this blog, coming from a Paleoproterozoic formation about 1.88 billion years old (Jirsa et al. 2008). It gets its name from the Mary Ellen Mine in the Iron Range. How did it get to where I live? Although it could have been a pebble in the subsurface, transported from the Iron Range by glaciers, it may instead have gotten here in gravel being used for the road work. The gravel would have been sourced from a gravel pit, which brings us back to glaciers again because the state's gravel pits are often in glacial deposits. In fact, in some of them you can pick out deposits left by different glaciers moving from different areas. Glaciers that passed through the Superior region brought agates south. Glaciers that passed through the Dakotas scraped up Cretaceous sedimentary rocks. (Cobban and Merewether 1983 commented on a fragment of phragmocone of a Cretaceous ammonite, Prionocyclus novimexicanus, found in glacial drift at the Brickyard.) The glacier that collected this pebble would have gone through the Range before depositing it.

This side of the pebble shows the characteristic red whorls of the stromatolitic layers with dark streaks between them.

In this photo I've put water on the surface, in case that makes it easier to see.

References

Cobban, W. A., and E. A. Merewether. 1983. Stratigraphy and paleontology of mid-Cretaceous rocks in Minnesota and contiguous areas. U.S. Geological Survey, Washington, D.C. Professional Paper 1253.

Jirsa, M. A., J. D. Miller, Jr., and G. B. Morey. 2008. Geology of the Biwabik Iron Formation and Duluth Complex. Regulatory Toxicology and Pharmacology 52(supplement to 1):S5–S10.