科研之友
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摘要
A unique circular feature of uncertain origin lies above the tree line on the eastern flanks of the Coast Range of British Columbia, Canada. It is composed of white, fractured, angular cobbles to boulders predominantly under 1 m across, arrayed in a slightly flattened circle nearly 50 m in diameter with the ring mostly about 4 m in width. The felsic granitoid lithology of the circle is unlike any in the immediate region, and no clasts of this composition occur within the circle. The debris rests on soliflucted soil containing rounded pebbles to cobbles of granodiorite that forms the regional lithology. The age of the circle is deglacial with post-glacial modification. Given the absence of similar lithologic units in the region and no obvious symbolic purpose for such a feature, an anthropogenic origin for the circle is improbable. Mechanisms for transporting the foreign rock unit to its final location, distributing fragments into a circle, and preserving it include flow of alpine and fringing continental ice masses, deglaciation, freeze-thaw cycles, and post-glacial solifluction and erosion. One large slab was either entrained within or fell onto a flowing glacier from some unknown outcrop at least 2 km from the site of the circle. During deglaciation, the insulating cap preserved the ice beneath it forming an isolated stagnant mass of ice. Freeze-thaw cycles likely affected the slab when it was on the surface of the ice, but it remained coherent, otherwise glacial flow or meltwater streams might have scattered loose clasts. Once the ice became stagnant, continued freeze-thaw cycles eventually created a rubble pile, which slid off the presumably symmetrical mass of ice to become arranged in a roughly circular ring. Having lost its protective cover, the ice melted and subsequent solifluction slightly modified the ring.
- 出版日期2011-12
