Environmental Engineering Reference
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the sand and dune deposits in column III toward the east and the continued presence of
the lacustrine silty clay deposit.
It should be noted that the presence of sedimentary deposits dipping in this example
represents the slope or contour of the original land surface and does not represent a struc-
tural cause such as faulting. However, some isostatic adjustment has occurred since the
materials were deposited and is evident from an elevation rise in the beach elevation of
approximately 1 m every 2 km toward the northeast. This rise is likely a result of the his-
torical glacial ice retreat from the region.
Additional analysis of facies also suggests the lake was once larger because a deposit
of lacustrine silty clay is present beneath the beach sand deposit in Stratigraphic
Column I. This suggests the lake was getting smaller and in a regressive stage before it
disappeared. The absence of top soil in Stratigraphic Column I and the presence of fill
material in Stratigraphic Column II also indicate anthropogenic disturbance.
Please do not get too comfortable—facies analysis is not always this straightforward.
Gradational or interpolated boundaries between geologic units require separate and care-
ful treatment. Gradational boundaries occur both laterally and vertically and are fairly
common in sedimentary deposits. Gradational boundaries should be identified and
explained in the legend and accompanying text according to the following guidelines:
• Nature of the gradational boundary
• Cause of the gradational boundary
• Extent of boundary
A sufficient facies analysis contains numerous stratigraphic columns with enough areal
coverage to enable an understanding of the region's depositional history. This information
allows for the construction of cross sections. Cross sections display the geologic deposits
without any breaks or gaps between the stratigraphic sections and help facilitate the con-
struction of a three-dimensional geological map (USGS 2002). Some degree of interpreta-
tion is usually necessary to construct a cross section, and these finer points are explained
in greater detail in the next section.
5.2.6 Developing and Constructing Cross Sections
A cross section is similar to facies analysis, but cross sections add levels of complexity and
additional information. A cross section represents a view of the subsurface as though it
was cut open and viewed from the side. Geologic cross sections are intended to be inter-
pretive and demonstrate the arrangement, relationship, structural composition, and depo-
sitional history of geologic units displayed in the cross section.
General guidelines for constructing a geologic cross section include
• Choosing appropriate horizontal and vertical scales
• Properly locating control points or points along the cross section where strati-
graphic sequences are known
• Ensuring the legend incorporates and explains each geologic material and feature
• Using appropriate symbols to identify each geologic unit
• Using appropriate orientations and landmark features
• Including vertical and horizontal scales
• Including a statement of vertical exaggeration
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