Phosphate rock in the Three Forks-Yellowstone Park region, Montana: Chapter G in Contributions to economic geology (short papers and preliminary reports), 1927: Part I - Metals and nonmetals except fuels

The region described is about 7,000 square miles in area. It includes most of Gallatin and Madison Counties, in Montana, and some contiguous parts of Idaho and Wyoming. Within it are several broad fertile valleys and a number of prominent mountain ranges. A considerable part is underlain by a bed of phosphate rock which occurs in the Phosphoria formation, of Permian age, and is elevated to the surface of the mountainous area. In most of the middle and southwestern parts of the region this deposit is about 2 feet thick; locally it is 3 feet or more. Outcrops of beds in which the deposit is more than 6 inches thick aggregate more than 200 miles in length. Toward the northeast the deposit thins out, and its limit in that direction is represented fairly accurately by a line drawn from Yellowstone Park northwestward through Salesville to Willow Creek, on Jefferson Kiver. Its thicker parts contain 45 to 65 per cent of tricalcium phosphate.

In this region the phosphate deposit is thinner and of poorer quality than in the adjoining parts of Montana on the west and northwest, where it is commonly 4 feet thick and contains 65 to 70 per cent of tricalcium phosphate. In southeastern Idaho the deposit is still thicker and richer.