ABSTRACT

Sphericity has been determined on quartz grains from beach sediments of northern New Jersey to determine its variation with size and genetic quartz type in a beach environment. Within the size range studied, -5 to +4, sphericities increased from a minimum of .59 at -4.5 (22 mm) to a maximum of .85 at +0.9 (0.55 mm), and then decreased again to .76 at +3.4 (0.10 mm).

It appears certain that both size, as such, and genetic quartz type, influence sphericity values; but their relative importance is uncertain. The size range in which the control of quartz type on sphericity appears best developed and most noticeable is between -1.5 and +1.0. In coarser grains, either sphericity when released from the parent rock, or mode of abrasion, seems to assume greatest importance. Coarser sizes consist very largely of vein quartz, which may have an inherently low sphericity. In finer sizes, sphericity decreases, apparently independently of quartz type.

Comparison with river pebbles from both the Aftonian Bridgeton Formation in northern New Jersey, and the present Colorado River (Texas) sediments, indicates that the beach pebbles tend to be more discoidal, and the fluvial pebbles more rod-like, in sizes coarser than -2.5 (about 6 mm). In gravel sizes finer than this, grains from the two environments are apparently indistinguishable by form. Sand grains of the beach samples studied have a rod-like tendency.