What a Geologist Sees - Part 10 [Original Post Date 11/24/07]

[A follow-up to the last post.] Up the hill from the old raceway grandstands - in a small cliff that normally marks the edge of the peninsula at Laurel Park - is this outcrop of old river gravels overlying saprolite, similar to what I wrote about in "What a Geologist Sees - Part 4". The previous examples of old river gravels over saprolite are on the west side of Duluth, also associated with the Chattahoochee River.

To refresh your memory, saprolite is the heavily-weathered "rotten rock" that has had its structural integrity destroyed by mineral dissolution (actually conversion to clays by hydrolysis). The term is usually used to describe former igneous and metamorphic rocks and traces of the orginal textures and structures must be discernible, otherwise we call it "residuum" - in this area, that would be Georgia red clay.

This hillside exposure of the old Chattahoochee River gravels overlying saprolite is an example of a "topographic inversion", i.e., this used to be the bottom of the river, in the lowest part of the valley. Now it is on a low hilltop, perhaps 1/4 to 3/8 of a mile from the river channel (prior to the filling of Lake Lanier in the late 1950s). A rough guess might be that these gravels are probably some 30-40 feet above the now-covered river bed.

On the opposite side of the peninsula, these river gravels have been eroded and the exposed shoreline is covered with the rounded pebbles and cobbles. I am considering doing a little gold panning, from sand trapped in dips in the exposed bedrock along the shoreline, as the Chattahoochee drainage basin includes Dukes Creek (near Helen, GA), which is a former gold-producing area. The idea is just to satisfy my curiosity. There are areas in which old river gravels have been mined for gold.

Again, I bring this up as a way to pique someone's curiosity. Most people don't give a thought to the river ever being anywhere but besides its current floodplain. What did the land look like before that 30-40 (maybe more) feet of vertical down-cutting took place, along with the lateral migration of the river? Questions such as these might arouse a kid's interest in science. In walking the drought-exposed shoreline, I am sure that numerous people every day see the gravels overlying the weathered bedrock, but I doubt that it really "sinks in" as to what it really means.

[I am so easily entertained.]