Geologic Cross Section Assignment

Draw a geologic cross section across line B-B’ on the given geologic map.  That is, construct a vertical representation of the rocks under the surface in a proper orientation, thickness and depth. 

You are given two cross sections, pens, pencils, erasers, and a rotated geologic strip along the section, with approximate thicknesses listed. 

The section should have the following units (some grouped)
Db
Dm
Dri/Dhl
Sk/Scy
Scl
Sc
Oj
Omb
Oe
Omc/Om/Obv/Ol/Olu
Ob

Please don’t draw on the geologic maps.  Use the handed out rotated geologic strip to draw on. 

You may talk with each other, but you must do you own work.  You absolutely are not allowed to copy or trace any other student’s or anyone else’s other cross section.

Due at the beginning of lab Mar 31.  There are two deliverables: the cross section and the answers to the following questions.

Answer the following questions first:

1. What is the dip direction on Little Mountain?  How do you know?
2. What is the dip direction on Jack Mountain?  How do you know?
3. Going NW from Little Valley across Little Mountain are you going up or down section?
4. Going SE from Little Valley across Jack Mountain are you going up or down section?
5. What type of structure is this from Little Mountain to Jack Mountain?  Give two lines of evidence that you know this.

Here are the steps for making a cross section:
1) Choose a line of section.
2) Construct a topographic profile along this line.
3) Build a stratigraphic column of thicknesses for the formations.
4) Post the formation contacts on the profile.
5) Post the dips on the profile.
6) Post any other general orientations of beds such as anticline and syncline hinges.
7) Draw in the beds attempting to honor orthogonal bed thickness.  
8) Erase often and repeat #7 :)
9) Ink the contacts and color in the final copy.

I have given you the topographic profile and other cross sections as a guide.  Use the stratigraphic thicknesses marked on the legend of the geologic map.  So, steps 1-3 are done for you.

For step 4, you make little ticks just above the topo of the section to indicate where the contacts on the map intersect the section.

For step 5, you project the dips into the section by looking at the strike from away from the section, and seeing where that measurement by posts on the section at the same elevation.  Then you draw the dip angle with a protractor as a little tadpole with a dot for the location of measurement, and line going down the dip.  You should have enough dips on the section to indicate the relevant structures, probably at least 10.  Do not project too far away from the line of section as these may not be representative along the section.

For step 6, use the same idea as you did to answer the questions above in order to figure out the structures across the line of section.  You indicate these hinges above the section as a guide for when you need to change the orientation of the beds in the section, guided by the dips you posted.

For step 7, extend the beds into the subsurface.  Try and maintain know approximate thicknesses as best you can, and honoring the surface location of the contacts between formations.  Use the patterns of anticlines and synclines to tell you the general orientations of the beds, but these do not have to be exactly the same dip.  Below the surface add all formations down to the base of the section. 

For step 9, after you have made a draft version of your section, use the light table in the back of the room to redraft this onto a new final section.  Ink in the contacts and dip tadpoles, and color in the formations.  Somewhere in the section or on the side indicate the different beds using the symbols, like Dm, Sc, etc.

Although this is interpretive, we all should come out with pretty much the same sections if we do it properly.  The positions of the contacts right at the surface of the profile should all be the same for everyone, thus be precise with the posting of the contacts.  Neatness and cleanliness count as part of the grade.  Work on this independently.