Making and Recording Geological Observations in an Outcrop
A rock outcrop is a patch of bedrock that is exposed at the surface of a planet. It can be a cliff face where the rocks are arrayed vertically in front of you, or it can be a horizontal patch on the ground that you can walk over. In either case, the rock layers (if they are layered at all) can be horizontally oriented, vertically tilted, or somewhere inbetween.
In an outcrop, you can note a lot of details, but usually it is better to start with the big picture and work your way in:
- Standing back from the outcrop, categorize the rock present by colour, bedded or not, thickness, how resistant they are.
Use the easily noted features. You can get up close later and investigate each category to see what type of rock it is, if all the instances really are alike, and if the different categories really are different from each other.
Identify if the rocks are in layers (bedding) or not.
Rocks that are in layers usually form when materials accumulate at the Earth's surface (depositional rock).
Rock beds usually represent an event of some sort.
These can be short and energetic (as for some sandstones) or long periods of quiet settling of materials (as for most mudstones and shales).
If the rocks are layered:
- How thick are the layers?
- How are the layers of different rock types arranged (alternating, clustered together)?
- Are there any patterns or trends in the layering that you can identify (do the proportions of rock types change vertically)?
- Are the upper and lower surfaces of each layer flat, undulating, irregular?
- Do the layers stay roughly the same thickness along their exposure?
- Are the boundaries between layers sharp or do they grade into one another?
Rocks that form less regular masses usually formed within the earth (intrusive) and conform to the shape of the cavity in which they formed.
- What shapes to the rock masses have?
- How large are the masses?
- Are they uniform internally?
- Describe any deformation that you can see.
- Moving in to look at the rocks close up we can look to see how the rocks formed.
Are they composed of grains or crystals?
Rocks composed of interlocking crystals suggest that the rock formed by cooling of molten material or precipitating from a solution.
Rocks composed of individual grains will have some gaps between between grains
Grains of material would be transported by wind or water and then settle out to eventually form the rock layer as they get cemented together.
- What minerals are present?
Minerals are naturally occurring, solid chemical compounds
Rocks are assemblages of minerals
At this point in the course your probably can only describe what you see rather than identify.
- Are there several different minerals or just one?
- What are the proportions of each mineral as well as relative sizes of the grains or crystals?
- Are the minerals randomly arranged or are there any patterns (banding, orientation or crystal shapes, trends)?
- Are there any identifiable structures within the rock layers.
These give valuable clues as to the type of processes and environment that produced the rock layer.
Are there any fossils within the rock layers (including fossil burrows and other indicators or animal activities)?
Are there any features on the upper and lower surfaces of the rock layers?
- sratches and other marks made by debris carried by a current (especially on the lower surface)
- evidence of animal activity
The goal of the process of description is to produce a written and graphical description of the rocks in an outcrop.