Structural and bedrock mapping along the York Rd, Lewis and Clark County, Montana

Miriam Barquero-Molina
Department of Geological Sciences, University of Missouri-Columbia
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This is a road mapping exercise with which students can get acquainted with the concept of detailed "structural mapping," combined with bedrock lithological mapping, and interpretation of structural data

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I have used this activity during a summer field methods course (field camp). Students were mostly juniors and seniors in Geology.

Skills and concepts that students must have mastered

Students need to know how to map lithologies, how to use a compass, how to read a topographic map, and must have taken a course in structural geology.
This exercise can be used as an introduction to structural mapping, or as an involved project in structural mapping, but the students need to at least be familiar with the basic concepts of structural geology

How the activity is situated in the course

It was a stand-alone exercise within a 6-week field course


Content/concepts goals for this activity

This exercise should reinforce the following ideas/concepts:
  • Asymmetry of parasitic folds on limbs of larger structures
  • Vergence of small folds as an indication of direction of transport along thrusts
  • Cleavage and bedding relationships on the limbs of larger folds
  • Way-up stratigraphic and structural indicators
  • Sketching of structures in the field
  • Plotting of structural data on a stereonet: poles to bedding, poles to axial planar cleavage, fold hinges and poles to axial planes; Pi-diagram (regional fold axis) and other structural relationships.
  • 3-dimensional visualization and spatial skills
  • Critical thinking
  • Pace and compass skills (optional)

Higher order thinking skills goals for this activity

  • Analysis of structural data
  • Formulation of hypothesis about the structural history of the area
  • Critical evaluation of structural data
  • 3-D visualization

Other skills goals for this activity

  • Learning how to meaningfully record structural data, and to draw good quality sketches of complex structural relationships

Description of the activity/assignment

The idea behind the assignment is to have students walk along the 2 mile traverse on the York Rd, NE of Helena, MT (north of the Canyon Ferry Reservoir), and construct a detailed geologic and structural map of that includes the different lithologies seen along the road, as well as reflects the complexity of the structures seen on the traverse.
Depending on the level of difficulty that the instructor sets for this exercise, students could be asked to:
  • Map lithologies and structures along the York Rd traverse
  • Measure structural features: bedding, cleavage, fold hinge lineations, fold axial planes
  • Sketch structural relationships seen along the road. Detailed and meaningful sketches should be emphasized.
  • Plot structural data on a stereonet and make structural interpretations based on the data collected

Determining whether students have met the goals

You look at their maps and fieldbooks, and evaluate their work in terms of data collection, valuable sketches, mapping skills, stereonet work and interpretation of structural data.

More information about assessment tools and techniques.

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