blackfoot-valley

The other main type of deposit on the valley floor is alluvium - that is, deposits of chiefly fine material spread repeatedly over the land by streams and forming mostly level bottomland. The flats formed by the shifting meanders and sloughs of Nevada Creek and the Blackfoot River, mostly north and northwest of Helmville, have al- ready been described. They include much of the Ranch headquarters area, in the drainage of Douglas and Cottonwood creeks. Those two streams join on the headquarters square of Ranch land, and their waters flow into Nevada Creek where the latter skirts the Ranch property line. (At one point the south bank of Nevada Creek nearly touches Ranch land, then joins the Blackfoot itself a mile beyond.) After a few meanders in the Blackfoot’s course, that stream starts cutting through, rather than depositing material, because of its steeper gradient and consequent faster flow, especially in the aforementioned “canyon” area. In the hills and mountain slopes on the western part of the Ranch, bed- rock is hidden under a thin layer of soil and exposed to view in just a few places, as is usually the case on only moderately steep slopes. Much of the bedrock in this area is igneous - that is, produced by volcanic action in ear- lier times. This is both extrusive (reached the surface as lava) and intrusive (which did not break the surface, and thus insulated, cooled slowly to become 29