The sandy expanses of beaches, deserts, and dune ﬁelds are loose aggregations of mineral grains, mostly quartz, created by geological forces that include plate tectonics and erosion. Wind and water sort this hard detritus so efﬁciently into masses of like-sized particles that we are able to speak of a sandbar (as distinct from a gravel bar) and other discrete forms, such as a mudﬂat, which is made up of same-sized particles of clay or silt. The smallest fragments of the earth’s pulverized rocks are called clay (each one measuring less than .004 mm—or forty microns—in diameter); the next largest are silt particles (.004–.065 mm); grains of sand, ﬁnally, measure between .065 and 2 mm. The word sand also refers to the loose aggregation itself, just as silt commonly refers to an infusion of ﬁne earth suspended in the ﬂow of moving water. On the continuum from the coarsest silt to the ﬁnest clay, the earthy substance gets progressively stickier.