Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 43

Individual grains or particles can be sorted by grain size—where the largest grains tend
to settle first, and by composition; where those grains with the highest specific gravity
minerals tend to settle first.
2.4.1.2 Grain Shape
Individual grains also vary in shape. The shapes of individual grains are characterized
by their degree of roundness; the degree of edge and corner removal; and their sphericity,
the degree to which the grain approaches the shape of a sphere. Roundness and sphericity
vary significantly and depend upon several factors, including composition, the method of
transport, and their distance traveled.
In general, roundness and sphericity increase the farther the material is transported
before it is deposited. Knowing (1) the shape of the grains and (2) that the largest grains
and those with the highest specific gravity settle out first during their deposition enables
geologists to make inferences about these deposits. For example, when examining a sedi-
mentary deposit, it may be possible to determine the relative distance sediment was trans-
ported, potential energy of the transport system, and method of transport.
2.4.1.3 Grain Size
Individual grains are also classified according to size and given specific names depending
on the grain size. For instance, a grain that has a size between 1/16 and 2 mm is classified
as a sand grain and a grain having a size between 4 and 64 mm is termed a pebble. The
Wentworth (1922) scale is the common method for classifying grain size, and the ranges
are listed in Table 2.2.
2.4.1.4 Composition
The grains in a sedimentary deposit tend to be resistant to mechanical or chemical weath-
ering. The composition of sand deposits is highly variable and depends upon the source
of the material. One of the most resistant minerals common in sedimentary deposits is
quartz, a dominant mineral of beach, river, and dune deposits (Boggs 2000). The quartz
content in modern-day sand deposits from major rivers and beaches in the United States
ranges between 67% and 99%, with the second most common mineral being feldspar. By
contrast, in tropical regions, a dominant component in sand deposits is limestone. Sand
deposits comprise approximately 25% of all clastic sedimentary deposits (Pettijohn 1975).
Figure 2.21 is an example of a quartz sand that is medium grain, well-sorted, with sub- to
well-rounded grains.
Composition is also used to help evaluate the degree of sorting in many sedimentary
deposits. Clay minerals are the predominate minerals constituting clay deposits, followed
by quartz, feldspar, carbonates, and iron oxide minerals, respectively (Pettijohn 1975;
Hillier 2003). Clay and silt deposits tend to be more erodible than quartz because they
contain higher amounts of organic material in addition to their mineral components of
micas and feldspars. Types of clay minerals include kaolinite; montmorillonite-smectite;
illite; and chlorite.
Clay minerals typically form over long periods of time from the gradual chemical
weathering of rocks, usually silicate-bearing, by low concentrations of carbonic acid and
other diluted solvents. The formation of carbonic acid involves the reaction between water,
such as rain and carbon dioxide in the atmosphere. As rain falls through the atmosphere,