EVOLUTION OF ALLUVIUM IN CENTRAL NEW YORK, USA

Every alluvial deposit contains rocks with individual stories, and this study seeks to develop quantitative relationships between shape, size, rock erodibility and tumbling history to aid in deciphering the rocks’s story. Streams incorporate rock fragments from bedrock exposures, and the recruits are usually very angular. Rocks tumble in a stream during large flows, interacting with each other and the channel bed. The impacts lead to rock disintegration. Sharp edges smooth during transport. Stream networks mix rocks with diverse transport pathways, so alluvial deposits tend to contain rocks of varying angularity.

This study investigates Paleozoic clastic and carbonate rocks in the northern Appalachian plateau of central New York USA. Representative rocks were gathered from streams and bedrock outcrops, then measured for shape, size, angularity, strength and erodibility. Rock elastic strength was estimated with a Schmidt hammer (10-60 MPa), and strength varied from < 10 to 55 MPa. Mohs hardness picks were used to estimate rock resistance to scratching, with values ranging from 1 to 6, and most falling between 2 to 3. To measure erodibility, equal masses of broken rocks and water were tumbled in a rock polishing mill of 30 cm circumference that rotates 140 revolutions per minute. A few hours to a few days of tumbling led to 10-80% declines in rock mass, depending on rock erodibility. Rock erodibility, measured as the decay constant in an exponential fit to fractional mass loss plotted against tumbling distance, spanned a couple orders of magnitude, from 10^-6 to 10^-4 per m. Shape evolution in a rock tumbler yields discs, rods, and spheres as possible final forms. Rock strength anisotropy controls shape evolution. Platy rocks are strong in two directions, and much weaker in the third. Round rocks are isotropic with respect to strength. Strength anisotropy determines which 3D shape characteristic (size, roughness, curvature, anisotropy, planarity) contains the most information on transport distance. However, curvature declines significantly for limestone, shale, and granite with tumbling distance. Shape measurements in 3D point clouds of particles could provide estimates of transport distance, if erodibility is known for the rock.