Gly 106

Spring, 1995

Laboratory 6 - The Third Dimension I

i.  Motivation.  Earth science information is conveyed not only in planform (map) views, but also in three-dimensions.  This is perhaps the most difficult aspect of the science to grow accustomed to.  Because the vertical heterogeneities in geological materials and properties are as strong as the horizontal, an ability to manipulate three-dimensional information is essential.

ii.  Reading.
Narrative portions of exercises 3, 4 and 5 in Peterson & Rigby "Interpreting Earth History."

iii.  Review concepts.
Material from previous labs

iv.  New concepts.
Stratigraphic columns, geologic cross sections, cliff forming and slope forming units

v.  Exercises.

1.  For the map units shown in the portion of the map of the (Oh, no!) Bright Angel Quadrangle, Arizona, construct a stratigraphic column.  Remember, stratigraphic columns show not only the rock units and their thicknesses, but also unconformities and the relative weathering of the units.

Show students parts of stratigraphic column

2.  A topographic map, structure contour map and a stratigraphic column can be used together to construct a geological cross section (Figure 1).  (An alternative and probably more common method is to use a geologic (unit) map with a topographic base.)  Follow the steps below to construct a cross section.

A.  First, construct a standard topographic profile along the line of section A-A' (Figure 1).  Transfer the line of section to the topographic map, then make the profile with a horizontal scale the same as that on the map (see the last lab for the numerical value of the horizontal scale), and a vertical scale of 1:1200.  Use 2300' as the minimum elevation on the profile.

B.  Draw in the structures on the structure contour map.

C.  Superimpose the elevation of the upper contact of the Millbrig/Big Bentonite on the topographic profile.  Where the contact is above ground level, use dashed lines.

D.  Draw in the other contacts on the profile to finish the cross section.  Assume that the units are of the same thickness throughout the map area.