Page 17 - Petrology of Sedimentary Rocks

sands is about 0.90; and the Colorado River at Austin, with its mixture of well-rounded,
Cambrian and Cretaceous quartz with freshly broken Llano granitic quartz has a ap of
I .30.

Pebble roundness can be measured more quantitatively by dividing the radius of
curvature of the sharpest single corner, by the radius of the largest inscribed circle (see
Dobkins and folk, 1970 J. S. P.)

Surface Features. As yet no way has been developed to measure these
quantitatively. Frosted surfaces are noticeable chiefly on round grains, although not all
round grains are frosted. Frosting may be caused by chemical etching (as by dolomite
replacement), by incipient tiny quartz overgrowths, or by aeolian abrasion (where the
frosting is due to minute crescentic percussion marks caused by the greater impact and
velocity in air). These may be distinguished by examining the grain with petrographic
microscope in water. Polished surfaces are caused by a fine smoothing of the tiny
irregularities and are ascribed to rubbing of the grains in water; thus they are the first
stage in rounding of aqueous sands. They are supposedly common on beaches but much
less so in neritic or river sands where the grains are not rolled back and forth so much;
but there is considerable question as to the validity of this criterion, and some polish
may form chemically (Kuenen). Much volcanic phenocryst quartz has naturally polished
faces. Unpolished surfaces have neither frosting or polish, and are dull or jagged
because of tiny angular irregularities or fresh fracture surfaces. They are chiefly found
in river or neritic sands and indicate lack of much abrasive action. Percussion marks
are found on pebbles (especially chert or quartzite) and indicate high-velocity impact.
Glacial action sometimes produces scratches or striae on soft pebbles like limestone.
The entire subject of surface features needs much more research to find out what
happens in various environments and to what extent the observed surface features are a
function of grain size.

As a possible explanation for the conflicting data on surface features and
environment, the following completely untested wild idea is put forth: all abrasional
environments produce all types of surface features, but on different sizes of particles.
In each environment the coarsest grains are frosted (probably by percussion marks),
intermediate grains are polished, and the finest grains are dull (unmodified). The size
ranges over which these surface features develop on quartz varies with the environ-
ment, perhaps as below:

size -4 -2 0 2 4
Neritic D U L L

River Frosted Polished D u 1 1
Beach Frosted Polished D u 11

Dune F r 0 s t e d Polished Dull

The most common size inspected for surface features is about medium sand,
which would explain the feeling that most dunes are frosted, most beach sands polished,
and river sands dull. Of course, rate of deposition, time available, and rate of supply
add complications, and some surface features are formed chemically. Mature desert
quartz often has a greasy luster caused by a minutely pimply deposition of quartz
“turtle-skin” overgrowths.

I I