GEOS 112 - Introduction to Earth Sciences
Making a Geologic Map
Before you come to class:
Preparation for this Lab:- Read the information on geologic maps and the colors used to denote the different rock units.
- Review the rock types you saw and described during last week's lab.
What to bring:
We will meet in the lab and from there spend the afternoon outside - rain or shine, so be sure to bring appropriate clothing. We will also climb along the lower cliffs below Summit Street, so make sure you are wearing sturdy shoes. Tennis shoes are OK, hiking boots are better. Long pants will protect you from poison ivy, thorns and brush. You will need:
- your lab book and a bunch of different colored pencils (several colors would be good)
- I will provide base maps, compasses, rock hammers and hand lenses
With our help you will map the position of rock outcrops found mostly along Zion Street. When you find a rock outcrop you should note the following:
- rock type (identify the rock using your notes from last week)
- location of the outcrop: chose a color for the rock type you are mapping and note its position on your base map
- How far does the rock unit extend? Where are its
boundaries?
The boundary between two rock units is called a contact. When you find a contact make sure you:
- enter its location onto your base map
- draw the orientation of the contact onto your map. You can use a compass to do this or you can orient yourself on the map and draw the contact freehand.
Once a certain pattern emerges you can connect the outcrops and contact information, mapping the distribution of rocks along the West side of campus.
In addition, measure the strike and dip of the rock units. Note the location of your measurements on the map (you can number them or label them in any convenient way) and write the strike and dip measurements into your lab notebook. By the end of this lab you should have a fairly neat table of several strike and dip measurements, listing the position of the measurement, the rock unit that was measured, and, of course the values for strike and dip.
The field version of your map should include: All the sites where you identified rocks and contacts. Your map should also indicate where you actually observed rock outcrops and where you simply infer the presence of a certain rock type. It should also include information about areas where rocks are hidden from view by buildings, or overlying sediment and soil.
An example: Rock formation A (orange) and rock formation B (blue) are found across the map area. The observed outcrops are represented by the polygons. This is where we know for sure the rocks exist. We also find a contact between these two rocks (solid contact line). This solid contact represents where we know a contact exists between the two formations. The dashed lines between the two formations represents where we infer the contact to be below the soil. Since we don’t have information about other contacts with other rock formations, the extent of the formations to the left and right is also inferred and is therefore represented with dashed lines. Coloring of the map distinguishes between where we observe the rocks to exist (dark coloring within the mapped outcrops) and where we infer the rocks to exist (light coloring outside of the outcrop polygons). Finally strike and dip symbols are used to describe the attitude of the rock formations. Remember – the strike is drawn at the correct compass angle (in this case the angle is 0° or 360°) and the dip angle is written next to the tic mark perpendicular to strike.
