Page 19 - Petrology of Sedimentary Rocks

beach; and (3) Stage (length of time available for modifying the particle). This is the
concept drawn from W. M. Davis, who used it for landscape interpretation. Concerning
Structure, bedding, schistosity and cleavage tend to make particles wear to discoids;
directional hardness may have some effect in minerals like quartz and kyanite. Original
shape of the particle may be somewhat retained, as in shapes of joint blocks, or in platy
quartz derived from some schists.

The effects of Process and Stage are complex, and there is insufficient data on
this aspect. In studying pebble shapes, most workers have used a grand mixture of
rocks, including schists, sandstones, thin-bedded limestones, etc. Of course such data is
useless; to get valid environmental data one must use isotropic wearing rocks such as
granite, basalt, or quartz. Further, the same geologic process (e.g. surf action) may
work to different ends with different sized pebbles, so pebble size must be carefully
controlled (i.e. use the same size pebbles for the entire study).

A study of pebbles in the Colorado River was made by Sneed. He found that the
main control on sphericity is pebble Iithology, with chert and quartz having much higher
sphericity than limestone. Smaller pebbles develop higher sphericity than larger ones
with long transport. Going downstream, sphericity of quartz increased slightly,
limestone stayed constant, and chert decreased. He found a weak tendency for the
Colorado River to produce rod-like pebbles in the larger sizes.

Dobkins and Folk (I 970 J. S. P.) found on Tahiti beaches, using uniformly-wearing
basalt, that beach pebble roundness averaged .52 while river pebbles were .38. Beach
pebble were oblate with very low sphericity (.60), while fluvial pebbles averaged much
higher spherici ty, .68. Relations are complicated by wave height, substrate character
(sandy vs. pebbly), and pebble size; nevertheless, a sphericity line of .65 appears to be
an excellent splitter between beach and fluvial pebble suites, providing isotropic rocks
are used. Apparently this is caused by the sliding action of the surf.

Roundness probably results from the chipping or rubbing of very minute particles
from projecting areas of the sand grains or pebbles. Solution is not held to be an
important factor in producing roundness, though some dispute this (Crook, 1968).
Impact fracturing of grains (i.e. breaking them into several subequally-sized chunks) is
not an important process except possibly in mountain torrents; “normal” rivers carry
few fractured pebbles, and beach or river sands only rarely contain rounded and
refractured grains. Rounded pebbles may break along hidden joints if exposed to long
weathering. The “roundability” of a particular mineral or rock fragment depends upon
its hardness (softer grains rounding faster), and the cleavage or toughness (large grains
with good cleavage tend to fracture rather than round; brittle pebbles like chert also
tend to fracture readily whereas quartz pebbles will round). On the Colordado River,
limestone pebbles attain their maximum roundness in only a few miles of transport and
thereafter do not get any more rounded. Quartz also becomes well rounded but at a
much slower rate, equalling the roundness of limestone after about 150 miles; chert
shows only a slight increase in roundness in over 200 miles of transport. Coarser grains
round easier than finer ones, because they hit with greater impact and also tend to roll
along the surface while finer ones may be carried in suspension. Aeolian sands round
faster than aqueous sands because the grains have a greater differential density in air,
therefore hit harder; also they are not cushioned by a water film. In experiments by
Kuenen, wind-blown quartz rounded much more rapidly than water-transported grains.
Beach sands also round faster than river sands because on a beach the grains are rolled
back and forth repeatedly. It is thought that little or no rounding of sand grains takes
place in rivers; pebbles get rounded rapidly in rivers, however. To get rounded grains
takes a tremendous time and a very large ependiture of energy. For example, the beach

I3