Laboratory Manual for Introductory Geology 4e

6.5.2 Grain Shape
Clast shape is a clue to the agent and distance of transportation. Clasts carried by
water or wind collide frequently with one another as they move. These collisions
knock off sharp corners and edges, eventually rounding the clasts. First, the grains
become subrounded, with smoothed edges and corners. Eventually, when they are
almost spherical, they are said to be rounded. Thus, the farther streams and wind
carry clasts, in general, the more rounded the grains become. The clasts also become
smaller as a result of these collisions. In contrast, grains that have not moved far
from their source tend to have sharp edges and corners because they have not had
the opportunity to collide as much; such grains are said to be angular. FIGURE 6.7
shows different degrees of roundness in two sedimentary rocks. Clasts transported
by glaciers are frozen into position and can’t collide with one another. Therefore,
glacially transported sediment tends to be angular.
6.5.3 Sediment Maturity
We discussed mineralogical maturity earlier in this chapter. We expand that concept
here to include textural as well as mineralogical changes in a broader concept,
FIGURE 6.7 Degrees of grain roundness in clastic sedimentary rocks.
Angular Subangular
Subrounded Rounded
(a) Angular clasts.
(b) Rounded clasts.
EXERCISE 6.6
Recognizing the Difference between Breccia and Conglomerate
Name:
Course:
Section:
Date:
Look back at the definitions of breccia and conglomerate. Based on these definitions, which of these two rock types
contains clasts that have been transported a longer distance? Explain.
6.5 INTERPRETING CLASTIC SEDIMENTARY TEXTURES
159