The art and science of geologic mapping involves the accurate depiction of contacts between rock units on a base map of some sort. This is what it’s all about — being able to draw a contact on a topographic or air photo! Obviously, this task is best done in the field where you can visually verify the location of contacts. Your ability to construct a reasonable geologic map in the field fundamentally depends on two things. First, you must know exactly where you are on a topo-sheet or aerial photo at all times — being lost is simply not an option! Second, you must know where you are in the stratigraphic section, which sometimes requires a little scrambling around (this obviously becomes easier with experience in an area). Unless you know where you are and what you’re standing on, it is impossible to draw a contact. Therefore, the overriding goal is accurate mapping of contacts and related geologic features (faults, etc.); it is not good enough to depict the “general idea” — we insist on accurately mapped contacts.
1. Keep track of your location on the map or aerial photo continuously as you traverse an area; typically, you will want to plan your traverses perpendicular to strike, thus crossing as many units as possible.
2. Orient the map in the direction that you are traversing: if you are walking east, then you should be looking eastward on the map. Always look ahead to where you want to go next on the topographic map, so when you get there you have already anticipated the topography.
3. When you encounter a contact between two mappable units, stick a pin through the map at your location, turn the map over, circle the pinhole, and assign a station number to it; then, enter the station number in your field notebook and note down everything you observe. Give the same location number to any samples collected. Typical notebook entries for a given station might include:
• Rock descriptions.
• Identification of formations (or other mappable units) at the contact
• Nature of the contact (disconformity; gradational; fault; igneous; etc.), Strike and dip, trend and plunge, etc.
• Estimates of formation thickness. Observations on topographic expression of rock units
• Structural cross-section sketch
• Stereonet sketch (to help visualize structural data)
• Sketch of complex outcrop relationships
Be sure to distinguish facts/observations from interpretations. Good field notes are richly illustrated with clean, clear sketches; one good picture is worth a thousand words!
4. Next, draw the contact on your map as far as you can confidently see it, either on the ground or the air photo (use a thin/sharp pencil with a soft touch, in case you need to erase it). Do not forget the Rule of V’s, but don’t exaggerate the contact migration in valleys. The contact location on your map should be accurate – not generalized or theoretical.
5. Then move on to the next contact. Remember, try to cover as much ground as possible each day and NEVER plan to return to the same outcrop later – chances are, you never will. Also, don’t stash your daypack expecting to return to it later. Once you get to a new spot, you may see something that dictates a different route than you planned on. Carry all your gear with you, all the time (this is a basic USGS rule of mapping).
6. To save time and energy, plan your traverse so it crosses the structural grain at a high angle (perpendicular to strike). Work back and forth over an area in a series of traverses, spaced according to the topography and complexity of the geology.
7. Take advantage of hilltops to check your mapping, refine your contacts, refine your traverse route, and predict contacts on the next leg of your traverse. Sometimes in rugged, high mountains, the view from a peak may be your only way to map cliffs, cirque headwalls, and other inaccessible places. In such instances, a good pair of lightweight field binoculars may be your best friend.
8. 0n your geologic map, depict the hinge lines of folds at the point of maximum curvature and be sure that your strike and dip data support the interpretation. Use medium-weight blue lines for fold hinges.
9. Use heavyweight red lines for faults and apply the appropriate design for different fault types (e.g., barbs in the hanging wall of thrust faults).
Use of High Resolution Imagery: High resolution satellite images are available from various sources including https://gisgeography.com/free-satellite-imagery-data-list/ at no cost for the entire earth. These can be a great help in locating contacts, which are quite often clearly visible as tonal boundaries. Just make sure, through ground trothing, that a change in tone corresponds to a mappable contact (formational boundary) and not simply a change in vegetation; if the tone change does correspond to a contact, then you may be able to extrapolate the contact well beyond the area of your traverse, and transfer these data back to your topographic base map. Working with stereo-pairs or images draped over digital elevation models can assist in visualizing the 3-d relationship of contacts with topography. When traversing about a ﬁeld area, pay attention to the plant cover that grows on different rock units. This will assist in your interpretation of images.
Background Information - Aravalli
Craton and the Delhi Fold Belt
This website is hosted by
Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)