Environmental Engineering Reference
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trend of a particular deposit. For instance, if a geologist were mapping a sandy beach
deposit from an ancient lake and recorded the trend of the deposit at several locations, the
geologist would be able to determine the size of the lake. The strike of a deposit, outcrop,
or other planar feature is represented on a geologic map as a short straight line segment
oriented parallel to the compass direction of the strike. The Dip is defined as the angle at
which the geologic deposit, feature, or structure is tilted relative to the horizontal plane.
The dip is represented on map as a line segment perpendicularly attached to the strike
symbol.
For unconsolidated deposits, evaluating strike and dip is much more difficult to mea-
sure since the sediments being evaluated may be loose. However, measuring the strike
and dip is still important and may provide valuable information concerning origin of
the formation and whether the unit has experienced any type of deformation since being
deposited. Evaluating a beach deposit from a former glacial lake provides an example of
natural deformation. Many former beach deposits from former glacial lakes are not hori-
zontal because the strike of many glacial lake deposits dip very slightly away from the for-
mer glacial front. This dip provides an indication of the amount of isostatic rebound that
has occurred since the glacier retreated and provides a basis for estimating the thickness
of the glacier. Measuring the slight angle produced by this process requires high accuracy
and precision since many deposits only dip 0.3-0.6 m (1-2 ft) per 1.6 km (mi) at most loca-
tions. In addition, measuring and evaluating strike and dip of unconsolidated deposits can
also be used to evaluate whether the unit has experienced any anthropogenic disturbance.
Figure 5.5 shows strike and dip of a set of layered deposits. Note the perpendicular dip
direction with respect to the strike direction. Figure 5.6 shows the strike and dip of several
geologic units and the corresponding information provided by strike and dip when con-
structing a three-dimensional representation of the data collected from a site. The numeri-
cal values on the figure represent the angle at which the units plunge into the subsurface.
Producing detailed intermediate maps showing the geology of an individual site is a
crucial step in the process of constructing a three-dimensional geologic map of an entire
region and watershed. Figure 5.7 shows a basic geologic map beneath a site where each
number represents a distinct deposit; Figure 5.8 illustrates a planar representation of the
Dip
FIGURE 5.5
Concept of strike and dip.
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