Motivation: For me there is something immensely satisfying in constructing a geologic map. They are a combination of observation, scientific logic and analysis, inferences, gut-feelings, and creativity. They represent a long-standing contribution to geology. Other geologists have expressed similar feelings. In a deformed terrane a crucial effort in the process of map making is the drafting of cross sections. It can be considered a test of the map pattern (most geologic maps have a significant component of interpretation since extrapolation between outcrops is necessary) and often leads to revision of the geologic map. Cross sections also allow for better conceptualization of the 3-D geometry of the structures, and thus aids in understanding the kinematic history of the area. For example, tectonic vergence (transport direction, often evident in the asymmetry) is immediately discernible in cross section, but less so from a map view. Also, if estimates of shortening are to be made drafting of a balanced cross-section is necessary, as will be described later in the course. Importantly, cross-sections suggest what exists in the firmament beneath our feet and beyond our vision - a 3-D extrapolation.

We are using a classic map and old map. It has a cross section displayed, but of limited extent. While this cross section can serve as a guide, you can and should be able to do better.

You should first spend time becoming familiar with the geology depicted on the map. What patterns are due to folds, what patterns due to topography? What types of faults exist? Are there any major unconformities? What type of stratigraphy is present? If not colored, adding colors often helps in understanding the map pattern. If you can identify a plunging fold axis, by looking obliquely at the map in the direction of the fold axis, you can get a cross section view (if topography is at a small enough scale it is not determining too much of the map expression).

Construct a cross section along a line of your choosing on the attached map. A cross section line should be chosen to show a maximum amount of information, i.e. it should cross major faults and fold structures, and should be perpendicular to the structural 'grain'. In this case you should draw a section from Salem Church through Dillsburg. Mark the cross section line on your map. Your cross section should include the following (and should be totally self explanatory); vertical and horizontal scale (note section lines), extrapolation of the structures into the air with dashed lines, extrapolation to a reasonable depth (think of any constraints you might be able to place on this), cardinal directions (top of map is north), and an explanatory key.

Several questions will confront you as you proceed. What colors to use? What fold style to use (angular, curved, parallel, similar, etc.)? The map itself will give you important clues to the structural style. How much does the map information constrain your interpretation? What dip to give to faults with little indication of dip? How far to extrapolate down to?

As far as steps in cross section drafting the following is a suggestion:

• choose cross section line (in this case it has been chosen for you).
• sketch out the structure in cross section in approximate and general terms.
• choose vertical and horizontal scales. 1 to 1 provides for no distortion, but sometimes with subtle structures a vertical exaggeration helps in depiction). However, you will need to compute apparent dips if your horizontal and vertical scales are not the same. The map you have is a 200% blowup of the original, which you also have.
• draft topographic profile. The topography is important for some maps and not as much for others. You often don't need to plot in every contour, but can use larger increments. Here every 100 feet should work.
• put in surface geologic boundaries along the profile and cross section line along with very short dip lines (at the appropriate angle) where available from the map. Don't forget about the concept of apparent dips. Put in faults (you can start out vertical if you don't have any information about fault dip, but be ready to reinterpret the fault dip).
• extrapolate units to depth using some consistent approach ( Busk method, parallel geometry, whatever). Start in less complicated and better constrained areas and work towards complicated structures. Thinking about thicknesses is important. Remember that if you are perpendicular to the strike of a unit you should see its true stratigraphic thickness, otherwise you will see some greater apparent thickness.
• work down to a depth where you have some constraint on what the geometry should be. In this map you can extrapolate down to at least the basement level using stratigraphic thickness information provided on the map.
• reinterpret and revise your cross section where inconsistent with the map or other data.
• extrapolate into the air with dashed lines.
• put on finishing explanatory touches.

Again ask yourself - is the cross section a) geologically reasonable, b) consistent with the map?

Graph paper is very useful as the cross section base. If I were doing this I would use Adobe Illustrator, starting with a scanned version of the map.

Be Neat! Don't hesitate to come to me with questions. You have two weeks to complete this one.

The above is a cross section of the north end of the Rio Grande rift in southern Colorado. Source of cross section: http://www.nps.gov/grsa/resources/geo_cross_section.htm

Link to Maryland geologic cross sections.

2005 - HDM