Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 30

Skinner 1974; Pettijohn 1975). The study of sedimentary deposits involves surficial pro-
cesses that include the accumulation, consolidation, deformation, origin, age, content, and
stratigraphy of sediments. Sedimentary deposits result from a sequence of processes char-
acterized by their removal from one location, some form of transport (e.g., down-gradient
by water or aloft by wind), and then final deposition in a sink . In sinks, sediments are not
at peace. Some may be subjected to heat and pressure and become metamorphic rocks; oth-
ers may move slowly with the ocean floor and become part of a subducting tectonic plate
and end up as molten magma in the mantle; and many will coevolve with the regional and
local biota to create unique ecosystems such as beaches, river deltas, and dunes. The vast
majority of sedimentary rocks were created in a marine environment as vast shallow seas
transgressed and regressed across the continental land masses.

2.3.1 Clastic Processes

There are three broad categories of removal processes involved with the formation of clas-
tic sedimentary rocks: weathering, mass wasting, and erosion (Figure 2.8). As shown in
Figure 2.8, the first process involved in the formation of a sedimentary clastic deposit is
the weathering (breakdown) of bedrock, which provides the sediment or clastic material
necessary to form a sedimentary deposit. Bedrock is defined as the continuous solid rock
of the continental crust (Flint and Skinner 1974). Once bedrock begins to erode, it forms a
material known as regolith —or weathered rock devoid of organic material.

2.3.1.1 Weathering

Weathering (process #1, Figure 2.8) is a destructive process by which rocks and minerals
are broken down through exposure to atmospheric agents such as air, wind, water, and ice
(Christopherson 2008). Weathering processes fall into two general categories: physical and
chemical. Physical weathering involves the fragmentation of larger rock into smaller pieces
by a mechanical process, without changing the chemical composition of the rock. Examples
of physical weathering include (1) abrasion, where rock is dislodged from the impact of
grains carried by wind, water, or ice; (2) fragmentation, from the downslope movement of

Processes

1

Material transport
downslope

Material transport
downslope

2 2

Bedrock

3 3

Processes

1. Weathering
2. Mass wasting (fall, slide, slump,
flow, creep) and weathering
3. Stream transport

Stream transport

Stream transport

FIGURE 2.8
Diagram showing the processes of clastic sedimentary rock formation: weathering, mass wasting, and sedi-
ment transport.