A problem was encountered in identifying the source of a distinctive rock type which occur locally as large subrounded to rounded clasts in conglomerates of the Crowsnest Formation, Alberta, Canada. The present day mountainous terrain was used as a model of the Early Cretaceous volcanic terrain. Observations of recent detritus in a stream which cuts the volcanics suggests that clasts of volcanic material may become rounded in less than two miles of horizontal travel. Abrasion of the larger clasts by the smaller detritus while the larger clasts are imbedded in the stream bed is inferred to be an important cause of the short distance rounding.

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