Raindrop impressions
Raindrop Impressions are small craterlike pits with slightly raised edges that are the result of the impact of rain on sediment surfaces.[1] Sedimentary structures with similar appearance have been found, and cause some controversy over how they would have formed.
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Description
Raindrop Impressions are only a few millimeters thick and less than 1 centimeter in diameter.[2] They can be preserved as widely scattered impressions or in close proximity to one another on the sediment surface.[3] Raindrop impressions have two forms of preservation. They can appear as small craters, as previously mentioned, or as small raised bumps. If the sample has craters, it can be concluded that the top of the sample is being viewed because the rain would have fallen onto it making indentations. If the sample has small raised bumps, then it can be concluded that you are viewing the bottom of the sample. In this case the raindrop impressions were created in the sediment, which was then followed by more sediment deposition that filled the impressions, creating, what appear to be, raised bumps on the bottom of the upper layer.
Controversy
Small circular pit-like depressions mostly in fine-grain sediments have long been interpreted as rain-drop impressions. As early as 1850, however, some geologists have expressed the opinion that most, if not all, supposed rain impressions are made by air bubbles rising through sediments. Despite these views, the practice of attributing these impressions to the action of falling rain drops has continued and geologic literature is full with examples of such interpretations. [4]
Arguments against a raindrop impression origin include:
1) during shower activity raindrops cover the entire surface, whereas socalled raindrop impressions are generally scattered and few in number;
2) because the rain is falling "everywhere," raindrop impressions should occur equally on sand-sized material as well as mud, yet in the geologic record these impressions are largely confined to fine-grained rock;
3) even when raindrop impressions form in mud during a shower, further rainfall eventually destroys them, leaving a surface which is not helpful to further preservation of impressions;
4) many examples cited as modern or fossil raindrop impressions can be explained by air bubbles rising through the mud; and
5) the characteristics of any impression depend on so many variables they can not be used convincingly to demonstrate those impressions formed specifically by raindrops.[5]
Ultimately, research has shown that most raindrop impressions that are preserved in the rock record, would have to have occurred towards the end of a rain shower. The decreased number of impacts at this time accounts for the scattered patterns of the impressions, and the specific saturation of the sediment allows for preservation. It is difficult to differentiate between actual impressions made by rain and other sources such as gas bubbles escaping, or dripping from another surface, because the structures that are preserved have the same characteristics. While not all so-called raindrop impressions are considered to have formed due to the impact of raindrops, one can not ignore their existence in the geologic record.
Images of raindrop impressions
In these images the rock surface is showing craters made either by rain drops or some other mode of creation. impressions like these can also be preserved as raised bumps. If bumps are being viewed, one can conclude that they are observing the bottom of the bed. The impressions would have been made, followed by sediment deposition, which fills in the craters. At a later time the two layers would have come apart, leaving the bottom layer with craters on its top surface, and the top layer with raised bumps on its bottom surface.
Links
Bibliography
- ↑ Boggs, Sam. Principles of Sedimentology and Stratigraphy. Upper Saddle River, NJ: Pearson Prentice Hall, 2006. Print.
- ↑ Stow, D. A. V. Sedimentary Rocks in the Field: a Color Guide. Burlington, MA: Elsevier Academic, 2005. Print. Retrieved from "http://pluto.potsdam.edu/geologywiki/index.php/Raindrop_impressions"
- ↑ Boggs, Sam. Principles of Sedimentology and Stratigraphy. Upper Saddle River, NJ: Pearson Prentice Hall, 2006. Print.
- ↑ Moussa, Mounir T. "Rain-Drop Impressions?" Journal of Sedimentary Petrology 44.4 (1974): 1118-121. GeoScienceWorld. Web. 05 Nov. 2010. <http://www.geoscienceworld.org/cgi/georef/1975021194>.
- ↑ Metz, Robert. "Why Not Raindrop Impressions?" Journal of Sedimentary Petrology 51.1 (1981): 265-68. Journal of Sedimentary Research. Society of Economic Paleontologists and Mineralogists, Tulsa, OK. Web. 05 Nov. 2010. <http://jsedres.geoscienceworld.org/>.
