Sailing Stones

Winter is coming to upstate New York so I decided to take us to the desert for this month’s column, specifically to the large, generally flat dry lake beds of Death Valley National Park in California.  For decades scientists have observed large rocks strewn about the playa surface and speculated about how they got there.  Often they are found with long, sometimes curved tracks suggesting movement across the desert surface.

A solitary “sailing stone” on the Raceway Playa in  Death Valley.                        (from Google Wikipedia)

A solitary “sailing stone” on the Raceway Playa in Death Valley. (from Google Wikipedia)

In the past, many theories were proposed to explain how rocks, some as heavy as 700 pounds, could slide or sail across the dry lake bed creating a furrow or trail in their wake.  Hurricane force winds and muddy playa surfaces were suggested.  Slick algal mats present during rare wet periods and thick ice accumulations were also proposed, but experiments and models developed with these ideas could not duplicate the phenomena.  Furthermore, it did not seem reasonable that roving herds of pronghorn antelope had entertained themselves by pushing stones around while no one was watching.  Besides there were no hoof prints!

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Pleurodictyum

I have had an active interest in mineral collecting for several decades (five years in Arizona can do that to anyone), but have developed a growing interest in fossils since moving to western New York two years ago.  Having completed my first full season of mucking up creeks and stopping at roadcuts in the Silurian and Devonian strata of western New York I can announce that I now have a favorite fossil.

I realize many are, and for very good reason, enamored with trilobites.  Be they DiPleura, Greenops, Dalmanites, or Eldredgeops , those are certainly great finds and I will be more than happy to pick one up when I come upon it.  But I took a fancy to a simple tabulate coral species this summer:  Pleurodictyum americanum, a species first described by the German paleontologist Carl Ferdinand von Roemer in the late 19th century.  Note his original drawings in the featured image for this post (Roemer, 1876).

Perhaps it is the near perfect symmetry of the colonial coral that grabbed my eye.  With a rounded top and a fairly flat bottom they certainly look grand once clay and shale is washed from the polygonal corallites covering the surface.   Even better presentation results when multiple specimens of variable size are displayed together.  My interest was also sparked because “pleuros”, as I have come to affectionately call them, are not as common as horn coral or many of the brachiopods species that are ubiquitous at many sites in the Finger Lakes region, but they are also far from rare and when found they are typically complete and recoverable.

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