Abstract

A total of 503 glacial cobbles and boulders in seven series were measured and classified as to type of rock, shape, character and degree of surface markings, and the like. One hundred and fifteen ice jam roundstones of various sizes were likewise measured and classified, in five different series. No absolute difference appears. The chief general distinction lies in the greater maturity of development of the stoss-snub surface on at least a few glacial dobbles, as compared with the slight modification of previous shape in ice jam cobbles. Grid patterns, crude bruises, and curved striae are more abundant, relatively, on ice jam cobbles and boulders than on those due to glaciation. On the basis of these criteria, the striated cobbles of southeastern river terraces are believed to be of ice jam origin.

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