Geology of Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 26

Before we focus on sedimentary rocks, a brief overview of some basic geological prin-
ciples is required. These principles (uniformity, stratigraphy, and unconformity) provide
the necessary foundation for understanding the general characteristics of sedimentary
rocks in urban environments.
2.2.2 Principle of Uniformity
During the late 1700s, James Hutton, a Scottish geologist, doctor, and farmer was studying
the rock outcrops around Scotland. Hutton, influenced by the Age of Enlightenment, cast
aside the existing religious dogma of a very young Earth and developed the principle of
uniformity , which stated that the external and internal processes on and within the Earth
observed today have been operating unchanged, and at the same approximate rates, for a
very long time throughout most of the Earth's history (Hutton 1788). Essentially, the mean-
ing of the principle of uniformity is that the present is the key to the past. The significance
of this principle cannot be understated, because the recognition of this principle presents
the geologist with a great tool for exploring the history of the Earth. Geologists today look
at features within observable rocks and search for insights into how they formed long ago.
For example, in some urban areas, the presence of sand found near the surface today might
indicate the presence of a beach thousands or millions of years ago. As we will soon learn,
the shape of the sand grains and their orientation can reveal even more information about
the past and the current physical and anthropogenic impacts.
2.2.3 Principles of Stratigraphy
Stratigraphy is the study of rock layers or unconsolidated sediment and strata; particu-
larly their ages, composition, and relationship with other layers (AGI 1962; Christopherson
2008). In geology, the term strata refers to the layers of rock or sediment with internally
consistent characteristics that distinguish it from contiguous layers (Krumbein and Sloss
1963). Each individual layer or stratum is generally one of a number of parallel and origi-
nally horizontal layers that were deposited by natural forces and lay one upon the other.
Therefore, the study of stratigraphy of a particular area provides important clues concern-
ing its geologic history.
Stratigraphy relies on four basic principles (Anderton 1985):
1. The principle of original horizontality states that the sediments formed from sedi-
mentary deposits are generally deposited as horizontal sheets. An example of very
fine horizontal layering of a sedimentary deposit composed of layers of clay and
fine silt is shown in Figure 2.4.
This principle is important because it allows the geologist to distinguish between
disturbed or deformed layers. In urban areas, this principle is especially important
because so much of the near-surface sediments and soil layers have undergone sig-
nificant disturbance due to development or deformation from tectonic activity. Some
examples of near-surface disturbance include farming, constructing buildings and
foundations, road building, mining, and landscaping. The term deformation gener-
ally refers to both brittle and ductile processes such as faulting or folding caused by
tectonic activity. This type of deformation explains why, for example, sedimentary
rock deposited thousands of feet below sea level may now be found thousands of
feet above sea level. This type of uplift is almost always the result of plate interaction.
Erosion or removal of a portion of a sedimentary layer through the action of water