A METHOD OF MEASURING AND PLOTTING THE SHAPES OF PEBBLES. By CHESTER K. WENTWORTH

The studies of which this paper is a report of progress were begun by the writer at the University of Chicago in 1917 and have since been carried on under the auspices of the United States Geological Survey, both in the field and in the oflice. Since October, 1920, the work has been done in the department of geology of the State University of Iowa, with the continued cooperation of the Federal Survey. The writer is indebted for suggestions and assistance to numerous friends at all these places. He would be grateful for comment and criticism and particularly for suggestions of localities for stud}dng special conditions and products of pebble-shaping processes. In a brief ^but suggestive paper H. E. Gregory 1 says, "Attempts to establish criteria for distinguishing beach pebbles from those formed by rivers, glaciers, or wind or resulting from weathering in place have led to unsatisfactory results." He goes on to show that most of the specific characteristics ascribed to pebbles of different origin are produced only by special sets of conditions and that most of the generalizations which have been made are either untrue or so vague as to be unreliable for the discrimination of deposits in the field. It is true that in a broad way it is possible to distinguish on sight between river pebbles, glacial pebbles, and sand-blasted pebbles. On the other hand, the general separation of beach pebbles from river pebbles can not be made with certainty on the basis of any criteria yet set forth.