Sinuous traces lying in the troughs on the bedding surfaces of ripple-marked quartzites from the Belt group of the Coeur d'Alene Mountains in northern Idaho and the Big Belt Mountains in southwestern Montana are identified as mud cracks. Probable identity of these structures with those previously described as "trails" or "burrows" from the Huronian Ajibik quartzite of Michigan indicates that these "fossils" are also of mud crack origin.

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