Clay Concretions

In August 1984 I was camping with my family at Little River State Park in Waterbury, Vermont. Bordering on the camp ground is the Waterbury Reservoir which at that time appeared to have been drawn down significantly. As I wandered along to shoreline looking for a place to fish some curious looking pebbles caught my eye. They were in many odd shapes, some resembled animals others were geometric figures.

I spent about an hour looking through the debris and I picked up whatever looked interesting. They were all soft and appeared to be made of clay.
Pictured in this page are some of the ones that I found. (Click on a picture to see a larger view)

Formed during the last Ice Age about 10 to 12 thousand years ago, they are not of any great age geologically speaking. Sometimes refered to as "Clay Dogs" or "Clay Stones" (Loughlin 27), clay concretions are not uncommon and have been found in abundance in a number of locations in New England(Arms 237, Loughlin 27) including along the Connecticut River and Button Bay State Park in Vermont.

While these objects appear to be cast from clay, in fact they also composed of 40 percent or more of Calcium Carbonate (Arms 239) which cements the clay particles together.

The manner or mechanism by which clay concretions are formed is still not well understood (NRCS). Some have suggested that concretions are formed around a nucleus of organic matter but often there is no evidence of that.

The Slurpee Theory of Calcium Carbonate Concretion Formation

My theory is that these concretions are the product of pockets or bubbles of carbon dioxide and methane gas created by the decay of organic matter sandwiched between heavy layers of calcium rich clay slurry. These trapped gases while under great pressure from the weight of the overlying sediment and perhaps a thick sheet of ice, created a super saturated solution of carbonic acid which dissolved calcium carbonate in the surrounding strata. When bubbles of methane and undissolved carbon dioxide found a path to the atmosphere, the space filled with a carbonated slurry which cooled suddenly due to the rapid gas expansion. The sudden freezing of the mixture and the loss of dissolved carbon dioxide caused the calcium carbonate or lime to precipitate out, thereby cementing the clay particles into the approximate shape of the cavity in which it was formed.

Works Cited

Arms, J. M. "Clay Concretions of the Connecticut River" Canandian Record of Science, The Vol. IV. No. 5 Jan. 1891 pgs. 237- 241
Google Books Accessed Apr 27, 2011

Loughlin, Gerald F. "The Clays and Clay Industries of Connecticut" State Geological and Natural History Survey Bulletin No. 4 Hartford Press 1905 p.27
Google Books Accessed Apr 27, 2011

NRCS-Natural Resources Conservation Service "What is a Claydog?" Connecticut NRCS - United States Department of Agriculture n. pag. Accessed Apr. 28, 2011

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