This is Saskatchewn clay country, a broad valley where the eagles fly. The white layer is called "The Whitemud Formation".
It itself consists of many layers.
Ravenscrag, Saskatchewan is an isolated hamlet in a large valley that has been cut from the prairies above by the Frenchman river. The river has revealed dozens of layers of sedimentary clays far below the surface, especially in the badlands sections of the valley. The "Whitemud formation" is an aptly named group of layers, it forms an obvious stripe that can be seen along the bottom of the valley edges. These layers range from highly plastic and refractory to balanced stonewares to silty low-melting to a sandy kaolin. The layer that yeilds Ravenscag Slip is the thickest and most consistent section of the formation.
This is Montana clay country, we extract 3 meter thick section of moderately plastic high iron clay from a long sloping hill.
This is near Grassrange, Montana. Nature has been kind to potters here. The reserves are limitless and consistent, you can dig anywhere and find the same material.
Alberta Slip is a product of Plainsman Clays and is mined in North America's premier "Clay Country" (near Ravenscrag, Saskatchewan, Canada and also in Montana, USA)