Why Clay is Dirt Cheap in my Neighborhood

I noticed a long time ago that, everywhere I dug at my place In Pennsylvania, I got clay -- not the nicest material for gardening. I fought it long and hard and finally got a good garden. Now, however, I want clay for my cob building project. Here is a quote from http://www.quoits.info/clay/clay.html that I find interesting:

Among the farmlands of Lancaster County, Pennsylvania, the soil is rich and fertile, and excellent for growing crops. But another favorable feature of this ground... is the large amount of naturally occurring clay soils. This clay was most likely formed from sedimentary deposits of the Susquehanna River on the western edge of the county, and other small creeks that flow though the area on their way into the river. Water is a natural producer of fine silt and clay, so many soils near these waterways can be composed almost entirely of clay. To find naturally-occurring sources of clay, you should first look in areas like this. If you can gain public access to property along streams or rivers, look along the stream banks and the ground just above them.

Well, I have a stream, so that explains it. The article also lists a source for the dirt-cheap clay, assuming I want to buy by the ton (and I do!)