Page 16 - Petrology of Sedimentary Rocks

scale: under .60, very elongate; .60-.63, elongate; .63-.66, subelongate; .66-.69,
intermediate shape; .69-72, subequant; .72-.75, equant; and over .75, very equant.
least
project i on
Actual lona widt h

least
projec tion
length

RILEY SPHERICITY
/
LEAST PROJECTION ELONGATION
Roundness was first quantitatively measured by Wentworth, who used the curva-
ture of the sharpest corner. Later, it was defined by Waddell as the average radius of
curvature of all the corners divided by the radius of the largest inscribed circle. This is
impractical to measure, though, and now roundness values are obtained by comparison
with photographic charts for sand grains (Powers). A perfect ball has a roundness of
1.0; most sand grains have roundnesses about 0.3-0.4 on the Waddeli scale. Use of the
Powers roundness images for sand grains is facilitated by a logarithmic (rho, p) scale in
which the limits of the very angular class are taken as 0.0-1.0, of angular I .O-2.0,
subangular 2.0-3.0, subround 3.0-4.0, round 4.0-5.0, and very round 5.0-6.0~ (Folk). On
this scale, perfect balls have a roundness of 6.0~ and most sand grains have average
roundness about 2.5~ (subangular).
largest
inscribed
circle

The concept of roundness sorting (uniformity of roundness) may be expressed by
the roundness standard deviation, op. This can be determined by plotting the roundness
data as a cumulative curve and determine op by the intercept method, as is done in
grain size curves. Plotting of these values for many samples gives the following limits
for roundness standard deviation: under 0.60, very good roundness sorting; 0.60-0.80,
good; 0.80- 1.00, moderate; 1.00-I .20, poor; over I .20, very poor roundness sorting. St.
Peter sand, uniformly well rounded, has up under 0.60; the average value for Recent

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