Identifying Outcrops

For today I want to talk about rock outcrops but more specifically just what to look for in an outcrop so you can determine various interesting things about an area. For this post I'm going to be essentially referencing an outcrop I recently went to with my Sediments and Stratigraphy class for a lab exercise.

When you get to a rock outcrop you should first just look around at the general area, this can tell you some interesting facts right off the bat. First off, from afar, you can get a general idea of the different bed sets. Basically all a bed set is, is a layer of rock (in my case sedimentary) that was deposited. You can then have multiple bed sets on top of each other with clear markings to where they begin and end which of course have been deposited at different times and in different environments. The general shape of the overall bed can give you a clue as to the main depository environment. Some bed shapes are; tablet, wedge, trough, lunate trough, lenticular spar, cylinder, and lens. Most of these look like how they sound or what their word root describe, such as lenticular spar looking sort of like an eye. All these basically describe how the sediment was deposited, for example rivers usually form lenticular shapes and therefore an outcrop in a lenticular shape was most likely deposited by a river.

The next thing you would want to do is actually get up close to the outcrop and examine it. You can determine many things from various factors in the rock. Grain size is a huge factor and range from clay and very fine silt (.004 mm/.008mm in diameter) to boulder gravel (256 mm in diameter). The size of the grain sizes can tell you how fast a river was moving when this was deposited as faster currents will usually make smaller grain sizes due to weathering. You can then look at the degrees of rounding of the grains which range from very angular to well rounded which also clue you in about the river as faster currents or river sections more downstream will have more rounded grains. Lastly you can look at the sorting of the grain sizes which vary from very well sorted to very poorly sorted. Again this ties in directly to the river deposition.

Other interesting factors that can help you learn more about an outcrop would be sedimentary structures and a quick biotic analysis. Sedimentary structures would include cross-bedding which are lines found in the rocks which can show ancient river current direction as well as help in determining the tilt and orientation of a bed. As for a biotic analysis, all I mean by that is to search for any fossils or plant material in the rocks. These can help you greatly in pinpointing the exact era an outcrop was deposited in and help in a stratigraphic log.