Class 9 Cross Section Exercise

Objectives: Make an interpretive geologic cross-section, project the saphire-bearing unit, and draw the isograds.

Procedure:

Download the geologic map “Saphire Ridge Mapping Project”. You will construct a geologic cross-section along the section line A-B. Make the cross section on gridded mylar or tracing paper if possible, or plain paper.

Part 1: Make the topographic profile:

1. Draw a line about one inch above the bottom of the gridded mylar, parallel to the bottom edge of the mylar, and the exact same length as section line A B. At point “A”, draw a line perpendicular to the baseline, 6 inches long. Do the same for point “B”. Label these vertical axes of the baseline “A” and “B”. They represent the vertical dimension of elevation. Make a tick mark on the left vertical axis at the point where the baseline intersects. Label this tick mark 1000 ft. Use a vertical scale of 1 inch = 1000 feet to label tick marks at 2000 ft, 3000 ft, 4000 ft, 5000 ft, and 6000 ft.

2. Register (match the endpoints) of the baseline with the endpoints of the cross-section line. If you are using regular paper instead of mylar or tracing paper, you will need to fold the page along the cross-section line.

3. Notice each point along the line where a contour line crosses the section line. At each point, make a tick mark on the baseline and label it with the elevation of that contour line.

4. Using the elevation scale on the left, place a small dot directly above each tick mark, at the point representing the correct elevation at each tick mark. Use the grid lines to maintain accuracy, or use a drafting triangle. For example, on the south end of the section line, the first contour line to cross the section line is at elevation 4200 ft. Place a dot 3.2 inches directly above the tick mark, which will match the same height as the 4200 elevation mark on the vertical axis on the left.

5. When dots are placed above each tick mark, connect the dots with a smooth curve. This curve represents the “topographic profile” of the surface (where it is intersected by a vertical plane).

Part 2: Add the geology:

6. Again, match endpoints of the cross-section base line with the endpoints of the cross-section line on the map. At each point where the baseline is intersected by a geologic contact line, place a tick mark. Label each side of the tick mark with the unit designator for the map unit. Use the gridlines or a drafting triangle to find the point which lies directly above each tick mark and on the topographic profile. For example, the first contact which intersects the baseline is the one between units “jhg” and “ha”.

7. Notice the foliations near the section line. Make a small mark at each point along the baseline where the foliation is mapped. Label the mark with the dip angle from the map. The average dip is 10 degrees.

8. Assume the contact surface between each map unit is parallel to the regional foliation. Draw lines on the cross-section to represent the contacts between each of the map units. For example, draw a line dipping 10 degrees to the right from the point on the profile line representing the contact between “ha” and “jhg”. There should also be lines dipping to the right 10 degrees and representing the contacts between all the other units. Use colored pencils to color the different map units shown in the cross-section.

Part 3: Add the claim boundaries:

7. Again, match endpoints of the cross-section base line with the endpoints of the cross-section line on the map. Notice where the base line is intersected by the north and south claim boundaries of the Dorothy claims. At each point where a claim boundary intersects the base line, place a tick mark on the base line. Use the grid lines or the drafting triangle to find the match for these points on the topographic profile.

8. All the schist units are pelitic. The Moonlight (impure) marble on the ridgeline contains epidote and tremolite. The Rialto schist contains almandine and the Carmen schist contains almandine and accessory corundum (saphire). The John Henry (impure) marble contains orthoclase. Refer to Figure 9 3. Is the Armadillo schist likely to contain sillimanite ?

9. The Carmen Schist unit contains gem quality saphires. Does this unit crop out at the surface on the Blue Sky claims ?

10. Draw a dashed line on the cross section which represents the metamorphic isograd (transition line) between the greenschist facies and the amphibolite facies. Draw the isograd between the amphibolite facies and the granulite facies.

Browse the information and images contained in the following web sites:

Metamorphic Rocks and Metamorphism:

http://www.em.gov.bc.ca/Mining/Geolsurv/Publications/InfoCirc/Ic1987-5/rocks/slate.htm

Photo of slate.