Friday, December 10, 2010
What a Geologist Sees - Part 4 [Original Post Date 8/19/07]
Have you ever noticed rounded river pebbles, on a hill-top or on some sort of plateau? Not just a few scattered pebbles, related to landscaping, but widespread occurrences and hillside exposures of seemingly intermixed soil and pebbles? On top of a hill?
How far did your sense of curiosity take you? Have you considered that "rivers move" over time? I am sure that many non-scientists have learned a little about how rivers meander and migrate, vis-à-vis the Mississippi and other rivers with broad valleys. But how often do we stop and think about the hilly terrain around us and how it changes over time?
This particular Topozone map shows the area in which both of these exposures were found. Notice the present location of the Chattahoochee River, approximately 1/2 mile north and west of this portion of Peachtree Industrial Boulevard in NW Gwinnett County, GA.
The upper photo area (Site 1) is now covered by a housing development. The site was approximately 200 yards due North of the intersection of Peachtree Industrial Blvd. and Abbotts Bridge Road, which crosses the Chattachoochee River. The present day Chattahoochee River channel is approximately 40 - 50 feet below the elevation shown at the highway intersection. From the lower left quadrant of the map, where Peachtree Industrial Blvd. enters the map, I have traced gravels for about 2/3 of its extent on this map, on both sides of the highway, to about the midway point between "Industrial" and "Blvd" notations on the map.
Exposures can be on eroded hillsides, in roadside ditches, creek valleys, and in construction sites. Because of present-day development of the area, it is difficult to trace the gravels for a greater distance to the southwest along P'tree Industrial Blvd, off of this particular linked map.
The lower photo area (Site 2) can be found in a small creek valley adjacent to Peachtree Industrial Blvd, probably 1/4 mile from Site 1. If you look at the linked map (if it works), in the lower left quadrant of the map, you will notice a series of long, parallel buildings (a storage area). Site 2 is across the small creek and up a side-valley. There are other exposures of river gravels overlying saprolite along this creek, directly across from the storage area site.
[While on the subject, saprolite can be described as "rotten rock", i.e., rock that has been so totally chemically-weathered, it has lost all of its structural integrity and can be crumbled by hand, though you can still see structures and textures in the outcrop.]
In both cases, the upper surface of the saprolite was once the eroded bottom of the river, which was then covered by the river sands and gravels. The contact is easily seen in the Site 1 photo, while at Site 2, the contact is shown by the dashed line. At one point, this essentially was "the lowest point in the valley". The present-day course of the river was, at that time, upland areas that had not yet been eroded by the lateral migrations of the river, as it also cut into the Piedmont soils and saprolite.
[In the Site 1 photo - "Poorly-sorted" refers to the wide variety of grain sizes, ranging from clay size to coarse-gravel sized particles. This is normal for mature rivers in this particular setting. The "Wentworth Scale" is one way of classifying grain sizes. If you have been to the ocean (or have seen sand dunes), where the sand particles are essentially all the same size, that is classified as "well-sorted".]
At the margin of the lower left quadrant of the map, where PIB "leaves the map" is one of our favorite local Mexican restaurants. If I am sitting in the restaurant and point out to those at the table that river gravels underlie the restaurant and all that is within sight, they might find that interesting, for the moment, but they will never see the fascination with trying to understand the nuances of "where the river used to be" versus where it is now. And other than the momentary "Wow, I didn't know that.", more thought will not be given to the subject.
That is why geologists are notorious for "talking shop" when we get together at parties and such. Cause almost no one else finds this stuff fascinating. Maybe that is why we become more eccentric as we get older (or maybe as our brains petrify). Maybe this is why we talk to ourselves (aside from teachers "practicing" their lectures).
There can be instances where tracing old river channels (and related sediments and sedimentary rocks) - in the subsurface - can be useful. Sometimes there can be mineralized zones associated with old river channels (which once-covered) served as conduits for the mineralized fluids. Sometimes the porosity and permeability of the sediments/sedimentary rocks may make them suitable for aquifers or oil reservoirs. If such river sediments lay beneath a proposed landfill, the sediments might serve as a conduit for leachate (landfill leakage) to reach an aquifer, thus rendering the site unsuitable for landfill use.
[Oops, I did it again. Yammered on for too long.]