This is from a quarry mining the Whitemud Formation in southern Saskatchewan. This layer is extracted from the top of a hill at the bottom of a valley. It is more than 50 meters below the table land above. The lumps are extremely dense and very heavy. They exhibit this horizontal layering, a clear indication of the sedimentary nature of the deposit. When I see this I know the clay is exceedingly fine particled. There are flecks of high-carbon material and some tiny iron particles. This lump is quite wet, about 12% water. When it dries out in breaks down into thousands of pure-white pieces, these slake quickly in water to create a creamy smooth slurry from which I can easily sieve out the carbon and iron particles.
Mother Nature's Porcelain - Plainsman 3B
How to Find and Test Your Own Native Clays
Some of the key tests needed to really understand what a clay is and what it can be used for can be done with inexpensive equipment and simple procedures. These practical tests can give you a better picture than a data sheet full of numbers.