Abstract: Several layers of dark green-yellowish brown hard clay in the Yangtze delta can be used as mark layers in the stratigraphy correlation due to their typical characteristics and widespread distribution. Among them, the first hard clay is considered as the boundary between Pleistocene and Holocene, and also the bearing course in engineering geology. The deposition of the FHC(first hard clay) has long been the focus of argument of whether aeolian or aqueous, fluvial or lacustrine.
Grain size is the most fundamental physical property of sediment, and grain-size analysis draws statistical indexes and geological information from miscellaneous data of those individual samples. Through systematical analysis of the FHC samples of Hemudu core in the Hangzhou Bay region and comparisons of grain-size distributions of Xiashu Loess, typical loess and paleosol from Mangshan and Duanjiapo profiles, and from fluvial and lacustrine facies samples, it is found that the grain-size characteristics of the FHC samples are very close to those of the lacustrine samples, while the Xiashu Loess and typical paleosol show similarity to some extent. The FHC deposition in Hemudu core in the Hangzhou Bay region occurred in fluvial-lacustrine facies with its parent material coming from fluvial flood-plain during the Last Glacial Maximum.