CHAPTER 3 Geology of New York State NYSDOT Geotechnical Page 3-10 June 17, 2013 Design Manual Ablation – the decrease in size and extent of a glacier by melting and evaporation. Alluvium - clay, silt, sand, gravel, or other rock material transported by flowing water and deposited in riverbeds, estuaries, and floodplains, on lakes, shores, and in fans at the base of