ANALYSIS OF THE RELATIONSHIP BETWEEN SHAPE, HARDNESS AND ERODIBILITY WITHIN CATSKILL STREAM EROSION OF A RETREATING WATERFALL SYSTEM

Rock erodibility is an important part of landscape evolution, and could be useful for understanding the development and evolution of retreated waterfalls, such as Kaaterskill Falls in the Catskill Mountains in New York state, USA. This project determines rock erodibility with rock tumblers and a Schmidt hammer, which characterizes rock elastic strength. Rocks were collected around Kaaterskill Falls, and then measured for rock strength using a Schmidt hammer. The rocks were then broken and subjected to tumbling in a rock mill. The mass of particles larger than granules was measured during the course of milling to determine mass loss rates. In addition, multiple photographs were taken of the particles to create 3D point clouds using structure from motion software. Zingg shapes were determined for the particles. After measurement, the particles and pulverized material were returned to the mill for continued evolution. Two rock types were sampled around the waterfall. They include a moderately indurate red shale and a moderately indurate sandstone. A sample of red shale was collected from a bedrock outcrop and from the stream channel. The sandstone sample was extracted from a stream channel.

The red shale exhibited variable erodibility, with cobbles in the stream showing lower erodibility (that is, greater strength and resistance to erosion) than chunks derived from a bedrock outcrop. Zingg classification revealed that the shales reduced in blade frequency over time while the sandstone increased. The shales showed an increase in disc frequency with tumbling duration, while the sandstone disc frequency decreased. Mass loss rates varied between rock types. Shales exhibited nearly constant mass loss rates, while sandstone showed a declining mass loss rate with time. The results provide some estimate of rock erodibility. The study also documents shape changes of alluvium due to chipping from impacts and surface wear from suspended particles in a fluid shearing environment. Such shape changes could be used to calibrate a function relating shape change to distance traveled for New York alluvium.