Making your map:
You’ve hopefully now identified the unknown rock samples, and have age information (radiometric ages) on some of them. This week your goal is to make a geologic map and cross-section. The sample locations are given. The first step is to think about how to group these different rock samples together. The simplest rock unit grouping is to make separate geologic units for each rock type. You can also group multiple rock units together if you think that is best. To do this, draw a line around spatially associated rock types and assign a particular color to that rock unit. In addition, label this color with a particular rock type in the legend. Think about what type of contact might separate different rock units (Bedding, faults, intrusion and unconformity). For example, a bedded contact would be between two sedimentary units deposited on top of one another, whereas an intrusive contact would be between a granite and the pre-existing rocks next to it. Continue this process until you’ve mapped all of the sample locations and rock types into geologic units.
Drafting a cross-section:
A geologic cross-section represents a vertical slice into the Earth and allows you to show your interpretation of the geology at depth. In the blank cross-section you are given, the topography has already been drawn in. Note that the cross-section extends along the line A-A’ on the map. To draw the cross-section measure the distance to each contact line from A and then record that location on the cross-section topography line. The dip of the geologic unit will tell you the angle you need to draw the contact line. For example a geologic unit with a dip of 0 deg. will be horizontal in the cross-section, whereas one that is dipping 90 deg. will be vertical. Use the same unit colors you used on the map.
Lab Report Instructions:
Guiding Question: What does the geologic map and cross-section tell us about the geologic history and evolution of our particular area? What additional information would you like to have about the area or about the rocks?
Hypothesis: After you have begun to examine the map and cross-section, but before you are completely done, make a hypothesis about the geologic structure or history of the map area. Think about what further information you would need to test that hypothesis.
Final questions to think about:
What kind of contacts exist between the geologic units? Could any of the geologic boundaries be something other than a bedded contact?
Can you identify a specific sequence of geologic events that must have happened in order to determine the observed geology?
What is the youngest unit? What is the oldest unit? Are there any significant gaps in geologic time (use the information from the previous geologic time lab)?
Are there any environmental changes that occurred over time in this area?
Have there been more than one igneous or volcanic events?
Turn in a 1-2 page lab report (absolutely no more!), your geologic map and cross-section.
As previously, this lab report should contain three labeled sections:
2) Methods and results and
3) Discussion and conclusion section.
The introduction should identify what concepts you were investigating and relate that to the guiding question. The methods and results should state what you did and the results without interpretation, and the Discusion / conlcusion section should analyze the results. The discussion section this week should contain your geologic interpretation of the area. Finally, conclusions should be clearly stated.