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No. 69: May-Jun 1990

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The enigmatic "mooring stones"

Cross-section of a 'mooring stone' hole
Cross-section of a "mooring stone" hole
Archeologists love to puzzle over pyramids, stone axes, and such straightforward productions of ancient man. In contrast, simple holes in boulders are hardly the things important scientific papers are written about. Yet, scattered about the Great Plains are some 300 boulders of very hard rock, each possessing the same rounded triangular holes. Surely such a phenomenon would pique some archeologist's curiosity!

The holes are made with high precision to the dimensions shown in the figure. They are 6 inches deep, plus or minus an inch. Holes with a rounded triangular shape represent a sophisticated drilling technology. Steel tools are high craftsmanship are indicated.

Even though the holes have been known for over a century, only amateurs have shown much interest. A few such enthusiasts have tracked down hundreds in Minnesota, North Dakota, South Dakota, Illinois, and the eastern seaboard. All of them seem to be located on present-day lakes and rivers and now-dry waterways. This marine affinity has led to the theory that they are "mooring stones," especially Viking mooring stones! In truth their real purpose is unknown.

How old are the holes? Weathering of those in granite suggest ages of at least several hundred years - well before the westward push of American settlers. The peculiar shape of the holes seems to rule out production by modern drills (usually round) for purposes of blasting or installation of surveyors' markers.

Another puzzle, probably related to the purpose of the holes, is the presence of large, smooth grooves on some of the boulders bearing the triangular holes.

The technique of "cam wedging" may lead us to the purpose of the holes. If one inserts a triangular shaft into such a hole and rotates it part of a turn, the shaft becomes firmly wedged in place. A rotation in the opposite direction quickly frees the shaft. It does seem that the mooring-stone hypothesis is consistent with cam-wedging.

(Olson, John J.; "'Mooring Stones': An Enigma Deserving More Attention," Epigraphic Society, Occasional Publications, 18:253, 1989.)

Comment. Before climbing on the Viking bandwagon, it is reasonable to ask whether the Vikings drilled the same holes in Europe.

From Science Frontiers #69, MAY-JUN 1990. 1990-2000 William R. Corliss

Science Frontiers Sourcebook Project Reviewed in:


  • "A sourcebook of unexplained phenomena is therefore a valuable addition to a collection of scientific literature. William R. Corliss has provided this in the past with his source books of scientific anomalies in several subjects, and now he has provided it for astronomy. He has done an excellent job of collecting and editing a large amount of material, taken in part from scientific journals and in part from scientific reporting in the popular or semi-scientific press." -- "The Mysterious Universe: A Handbook of Astronomical Anomalies", reviwed by Thomas Gold, Cornell University, in Icarus, vol.41, 1980

  • "An interesting, systematic presentation of unusual weather [..] This book is recommended for a general audience" --"Corliss, William R., Tornados, Dark Days, Anomalous Precipitation, and Related Weather Phenomena, Sourcebook Project, 1983.", revieweed in Choice, September 1983
  • "..the science is necessarily somewhat speculative, but Corliss's symthesis is based on reputable sources." -- "Corliss, William R. (Compiler). Lightning, Auroras, Nocturnal Lights, and Related Luminous Phenomena" reviwed by Joseph M. Moran, Univ. of Wisconsin in Science Books and Films, Sep/Oct 1983

  • "Before opening the book, I set certain standards that a volume which treads into dangerous grounds grounds like this must meet. The author scrupulously met, or even exceeded those standards. Each phenomenon is exhaustively documented, with references to scientific journals [..] and extensive quotations" -- "Book Review: The moon and planets: a catalog of astronomical anomalies", The Sourcebook Project, 1985., Corliss, W. R., Journal of the Royal Astronomical Society of Canada, Vol. 81, no. 1 (1987), p. 24., 02/1987