Disturbance

Disturbance of sedimentary deposits occurs constantly from the onset of sediment formation. There are several anthropogenic and natural types of disturbance, and w'hen conducting studies in an urban environment these should be described and evaluated.

Anthropogenic Disturbance

One common concern for any investigator evaluating shallow unconsolidated deposits is whether any disturbance has occurred since deposition. This concern is ratcheted up in urban and developed areas since anthropogenic disturbance often includes either excavation or filling. Often this type of activity can be readily observed—but this is not always the case. Typically, to evaluate the existence of anthropogenic disturbance in a specific area, geologists rely on a few' clues for assistance. Some of these clues include:

  • • Evidence of historical development
  • • Evidence of landfilling
  • • Evidence of grading or land surface disturbance
  • • Nonnaturally occurring debris located w'ithin sediments
  • • Evidence of excavation
  • • Lack of original deposition structures w'ithin the deposits in question
  • • Inconsistency or evidence of stressed or absent vegetative cover
  • • Topography inconsistent with surrounding area or region

Figure 2.47 is a photograph consisting of landfill material. In this case several metal containers were excavated indicating anthropogenic disturbance.

Naturally Occurring Disturbance

Disturbance can also occur naturally, although at times it may be difficult to determine whether the disturbance is natural or anthropogenic. For instance, spoils from dredging historically were spread over the land surface in low areas to enable the development of these areas. Dredging a river bottom and spreading the material over the land surface is accomplished by pumping a combination of river water and bottom sediment to the desired area of disposal. Using water as the transport agent for the

Example of anthropogenically deposited material. (Courtesy of Daniel T. Rogers.)

FIGURE 2.47 Example of anthropogenically deposited material. (Courtesy of Daniel T. Rogers.)

sediment may create some difficulty in identifying the source of the disturbance, because the water and this mixture of sediments and other materials can leave depositional structures similar to natural deposits. Therefore, special care should be undertaken when evaluating sediment layers in urban settings.

Examples of naturally occurring types of disturbance include the following (Pettijohn 1975):

  • • Diagenesis, typically involving compaction and lithification
  • • Erosional removal of a portion or an entire deposit (causing an unconformity)
  • • Bioturbation involving the disturbance of very-near-surface soil layers typically by worms and other macro invertebrates
  • • Vegetative disturbance from trees with large and deep root systems
  • • Differential compaction or subsidence; these events can obscure, bend, or offset original depositional deposits and sequences and make interpretation of deposition challenging
  • • Redistribution of sand deposits by wind along a beach, forming dune deposits often difficult to recognize because many dune deposits do not contain easily recognizable depositional structures
  • • Offsets in bedding planes or sediment layers from faulting or other tectonic activity. The results of tectonic offsets may make the matching of depositional layers impossible depending on the distance of the offset.
  • • Landslides along steep slopes or unvegetated hill slopes resulting from fire or flooding
  • • Chemical disturbance from the migration of water from the surface through the sedimentary layers; leaving behind a chemical precipitant or dissolving a portion of the original constituents of the sediments. Given the correct circumstances, this type of disturbance may lead to a karst topography.

The upper portion of Figure 2.48 shows an example of a disturbance likely originating from a naturally occurring source because the line separating the disturbed portion from the nondisturbed portion is not sharp, as would be characteristic of an anthropogenic source. Instead, the boundary is transitional and gradual in most areas, suggesting the disturbance was probably a combination of bioturbation or differential subsidence.

Late Pleistocene unconsolidated sedimentary deposit in the Rouge River watershed composed of sand (lighter color) and ash (black). (Courtesy of Daniel T. Rogers.)

FIGURE 2.48 Late Pleistocene unconsolidated sedimentary deposit in the Rouge River watershed composed of sand (lighter color) and ash (black). (Courtesy of Daniel T. Rogers.)

Summary and Conclusion

The geology of a watershed literally forms the foundation of where we live. Moreover, the geology of any urban region shapes the manner in which we live: our lifestyles; food production; natural resource availability; economic activity, and ultimately determines our standard of living and our sustainability. We have explored the different types of geological environments where urban developments are located, and discovered that most urban areas exist in geological settings of sedimentary origin. We have also discovered why the geology of urban areas is very complex and changes rapidly. In addition, we learned every urban area of the United States has a unique set of geologic deposits formed from more than two—and in some cases up to eight—distinct geologic processes.

We have explored a representative sample of the many types of geological environments of urban areas throughout the United States. What is abundantly apparent from this investigation is that many urban areas are located in geological settings dominated by sedimentary deposits located near water. There exists a complex, dynamic, co-dependent, and sometimes delicate relationship between the presence of water and geology. When urban areas are placed into the mix, these complexities and delicate relationships may be thrown out of balance. Although not always intentional, the resulting human impacts may alter our lives or the environment negatively, and in some cases catastrophically. What determines our sustainability is only in small part determined by urban location; most critical are the specific actions we undertake to understand and protect the urban environment.

Finally, we have briefly introduced the most important factor determined by geology at any location, and especially in urban areas: the presence and availability of water—the most precious resource required for living organisms and their continued sustainability. Robert Nace (1969), a noted global hydrologist once stated, “the story of the growth of civilization and science could be written largely in terms of human concern with water.” The next chapter explores water in the urban environment from a scientific perspective and encompasses a wide range of processes from its basic molecular structure and chemical behavior to its actions on the surface and below the ground.

 
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