DIGITAL IMAGE PROCESSING FOR GRAIN-SIZE AND GRAIN-SHAPE ANALYSES: UNDERSTANDING THE NGANDONG PALEOANTHROPOLOGICAL SITE FORMATION

The Ngandong paleoanthropological site in Central Java, Indonesia has significant impact on the models for human migration and evolution out of the African continent. Located on a fluvial terrace level ~20 m above the Solo River, paleoanthropological excavations at Ngandong have uncovered fourteen Homo erectus crania fossils that because of their morphology are possibly some of the most recently known occurrences of H. erectus in the world. However, this hypothesis cannot be substantiated by the results of previous studies at Ngandong due to a general scarcity of geologic knowledge of the Solo River terrace sequence formation. My study seeks to overcome the limits of these previous studies by taking a comprehensive, geoarchaeological approach to this paleoanthropological site by pairing sedimentary facies description with grain-size and grain-shape analyses using Retsch CAMSIZER® digital imaging technology. The Retsch CAMSIZER® has been commonly used in industry to ensure standard size and shape of various aggregates, such as coffee grounds and granola, but may also be applicable for grain-size and grain-shape analysis of geological aggregates within the 30 µm-30 mm size range. Benefits of using digital imaging for grain-size analysis over the traditional particle-size analysis (PSA) techniques, particularly sieving, include a shorter measurement time, higher accuracy and reproducibility, and the ability to save data in up to 1,000 different size classes in an easily-manipulated digital format.

In this study I use CAMSIZER® digital image processing for grain-size and grain-shape analysis of the fluvial deposits sampled from the Ngandong paleoanthropological site. Preliminary grain-size data confirm the qualitative sedimentary field descriptions in that there are four distinct fluvial deposit layers above bedrock that differ in grain-size mode and degree of sorting. In addition, the bone-bearing layers are the most poorly sorted within the site and show greatest variation in grain-size across the site, supporting hypotheses that the flows responsible for vertebrate bone deposition at Ngandong were special events that differed from the typical fluvial dynamics of the Solo River in that location.