Geologic Mapping I

Goals:

• Combine field observations of location and lithology to make a geologic map
• Understand the continuity of rock units under the topography between outcrops and at depth
• Understand the interpretive quality of geologic maps and cross sections

Skills:

• Determine location on a topographic map
• Infer location of rock types by elevation, strike and dip
• Construct a topographic profile and geologic cross section
• Work with vertical exaggeration on profile and cross section
• Describe rocks in hand specimen and outcrop

This field lab could stand alone or be part of a sequence of four or five field labs that introduce the bedrock geology of the local area. Depending on its position in that sequence, you'll need to allow more (or less) time for students to recognize and describe the rock types before they begin to map their locations. If you plan an initial lab where students describe different formations in outcrop, you can find tips in the teaching materials at this page.

The field part of this lab takes at least two hours, not counting transportation time.

Required supplies and equipment:

• Large-scale topographic map of field site
• Clipboards or map boards
• Compasses
• Hammers (if appropriate and permitted at the site)
• Graph paper (after the field lab) for drafting geologic cross-sections
• Students will need to bring pencils, colored pencils (optional), ruler/protractors, field notebooks

The best areas for this field lab are those with scattered outcrops (not continuous) of at least a few different kinds of rocks. Sedimentary rocks or layered volcanic rocks probably work best. Horizontally bedded rocks work just as well as deformed rocks; the idea is that students should learn to predict from the first exposure which rocks would crop out where in the field area.

The maximum area that students can cover in a single lab period is about half a square mile (less if there are many exposures to locate, map and describe).

The topographic base map for the lab should include only the mapping area plus such landmarks as roads, rivers, major trees, etc. A map covering a larger area can be used as reference. In some areas, using an expanded air photo base will be appropriate as a supplement or replacement of the topographic base map.

When completing their final maps, encourage students to use a darker shade of color in the areas where they actually saw rocks and a lighter shade of the same color in the intervening areas. This helps students understand the interpretive quality of all geologic maps.

Follow-up activities in class periods and labs can include comparing the geologic maps the students have made with published geologic maps (these don't need to be of the same area or region). You can also have a good discussion about the concept of a geologic "formation" after students have mapped and described a variety of rocks in the field.

Students complete a geologic map and cross section for this lab, including a complete key or legend with rock descriptions of various formations. These products can be assessed on completeness, soundness of extrapolations and interpretations, and understanding of the geologic context, as well as clarity of illustrations and writing.

Assessment Rubric (Acrobat (PDF) 59kB Apr5 05)