Urban Geologic Mapping - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 183

presentation of the information to intended users. Selecting an appropriate method or
combination of methods depends on (1) the level of complexity, detail, and scale of the
information and area being mapped; (2) the intended users, such as scientific or academic,
land-use planners, or the general public; and (3) cost.
Some common methodologies used to prepare the map include
1. Preparing a series of maps showing the regional development and evolution of the
landscape of a particular region. These maps tend to lack great detail because the
geographic scale is generally very large. The purpose of these maps is to convey
major concepts of regional significance.
2. Preparing several maps beginning with the deposition of the bottom or oldest
layer of sediment, and then continuing with the display of distinctive inter-
vals of equal time or significant geologic events of deposition. These maps have
advantages because they can be very detailed and focus on significant geological
events.
3. Adding geological processes that formed the sedimentary deposits such as stream
or river succession, lake elevations, or glacial advances or retreats. This type of
map combines the areal distribution of geological deposits with the methods of
their formation. Although this type of map is very effective at conveying pro-
cesses with geological deposits, it may lack desired detail depending on the scale
of the map.
4. Producing maps in three dimensions. Constructing maps in three dimensions
generally requires more time and effort, but has the benefit of providing better
data gap analysis and integrating landscapes into the geologic map.
5. Using GIS to combine geological maps with other digital data layers such as roads,
buildings, parks, utilities, drainage patterns, proposed developments, and popula-
tion. A composite map representing the overlay of some or all of these layers can
produce new information about the study area.
The most common methods are #3, #4, and #5, or some combination of each.
5.2.8.1 Succession of Maps
A succession of maps is one method used to decrease the time necessary to understand
the geological processes responsible for the development of sedimentary deposits in urban
areas. This technique presents complex processes gradually through the use of several
maps that incrementally add complexities and depositional processes. At a regional scale,
building layers showing different geological processes occurring at specified intervals
and/or depicting the sequences forming deposits can simplify the communication of geo-
logic information. These types of maps can also incorporate three-dimensional and GIS
formats to present the information.
An example of a succession of regional maps of the Great Lakes region is presented in
Figure 5.14a through h (modified from Hough 1958, 1963). This map succession shows the
regional development and evolution of the landscape for a particular region at a specific
time—the Great Lakes during retreat of the last glacial advance of the late Pleistocene.
These maps are general in nature and lack detail because their geographic scale is very
large.