Urban Geologic Mapping - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 165

5
Urban Ge ologicMapping
5.1 Introduction
The methods presented in this chapter provide the basic building block for under-
standing the geology and hydrogeology beneath urban regions. This chapter begins by
explaining the process of interpreting and integrating information from the environmen-
tal investigations presented in Chapter 4 with other sources. The chapter concludes by
combining this information with field mapping techniques to construct a geologic map
of a watershed.
Geologic maps tell a story of the history of a particular area, and their intended users
are geoscience professionals and the general public. Urban geologic maps present many
new challenges to geologists and are typically complex to construct. One of the key chal-
lenges is to create an easily understandable and usable geological map for members of the
nonscientific community. This should not come as a surprise to the geoscience community,
since it has long been one of the major goals to construct geologic maps with the ability to
clearly communicate scientific information to a diverse audience. John Wesley Powell, who
as director of the USGS stated in 1888, “maps are designed not so much for the specialist
as for the people, who justly look to the official geologist for a classification, nomenclature,
and system of convention so simple and expressive as to render his work immediately
available” (Powell 1888).
Effectively communicating complex information is joined by other unique challenges
when developing urban geologic maps. As many geologists studying the geology of urban
areas know, access and disturbance of the natural landscape present some of the greatest
problems when trying to develop a map or while conducting an environmental subsur-
face investigation. The geologist must be clever, tenacious, and opportunistic because a
large geological map base covering urban areas does not exist. Only recently has interest
in geologic mapping of urban areas become a focus in the United States (USGS 1998, 2009;
Great Lakes Geologic Mapping Coalition 2009). The unconsolidated near-surface geology
of urban regions has been largely ignored because urban areas are not in the spotlight like
areas with valuable natural resources or risk zones such as those prone to earthquakes or
volcanoes. Despite this lack of high-profile attention, characterizing, understanding, and
evaluating the geology at urban locations have been quietly conducted at tens of thou-
sands of sites throughout the United States, and these efforts collectively represent a mas-
sive information database of fundamental geologic knowledge.
The complexity associated with urban geologic maps is directly related to the charac-
teristics of the urban environment. As shown in Table 2.4, the vast majority of large urban
areas in the United States are located on unconsolidated sedimentary deposits. Most of
these areas are less than 2 million years old—young by geological standards—but com-
plex in nature by virtue of them having natural- and anthropogenic-induced unconformi-
ties as a common occurrence. These urban areas are also proximate to large amounts of
surface water and groundwater, and this water plays a fundamental role in functioning as