Wild Horse Plains lies in a circular valley at an elevation of 2,450 feet, drained by the Clark Fork River. Between 70,000 and 130,000 years ago, Bull Lake ice age glaciers dammed the Clark Fork River Valley and created Glacial Lake Missoula as waters from the Clark Fork drainage backed up to form a lake. When the ice dam broke, the Clark Fork River carried more water than the combined flow of all the streams of the world. In the early 1800's, Native American tribes traveled through the area, using the fertile valley to winter their ponies, harvest salmon, and hold great councils. Mountain men, trappers, surveyors, and map makers soon followed. White settlers began moving into the valley in the late 1860's, and in the following decades farming, ranching, and lumbering flourished. The Northern Pacific Railway arrived in 1881-1883, and the town grew in size and importance as businesses prospered and the name was shortened from Wild Horse Plains to Horse Plains and finally to Plains.