blackfoot-valley

the shapes of the most evident landforms in this area of Montana - namely, the mountains - periodically but relentlessly sculpting them by carrying off tons of material during mostly the warmer half of each year. When running wa- ter slows, as its slope becomes gentler, it is no longer able to carry the larger rocks, and deposits the finer material farther down. The Blackfoot River and lower Nevada Creek just north of the Ranch headquarters have in this manner produced a sizeable area of very flat land crossed by their many periodically changing meanders. The depositing of mud in fine particles - mostly silt and clay - over time raises the streambed level to a point that the stream is forced by gravity to change its course to seek its lowest possible level in a new me- ander. This happens again and again until a remarkably level floodplain is the result. In a small valley like this one, if conditions are right, such a floodplain can be produced within a period of only a few thousand years, sometimes even within a few hundred. The flats of this mini-floodplain, in a rough oval area five to six miles across and covering twenty plus square miles, can be best viewed from Mon- tana State Highway 141 just north of Helmville where the road follows the long ridge of a glacial moraine, giving an excellent view of the marshy floodplain with its scores of stream meanders and small oxbow lakes and bogs left over from former meanders. Driving along that road, it’s well worth a stop to con- sider this story told by running water that has built up the land rather than cutting through it. (It’s a safe bet that few motorists driving over this stretch of road give a thought to what created this land form.) From several outlook points along this ridge, the northern part of the Ranch is spread out in a pan- orama beyond the flats; and from the right spots along that highway, the build- ings of the Ranch headquarters with its conspicuous red silo can be easily seen in the distance. That ridge giving us our vantage point brings up another agent of depo- sition, which gives much of the valley floor a more conspicuous aspect of low hills - namely, ice. Although ice has had a major hand in sculpting the forms of the higher peaks seen to the northwest, down in the valley itself ice has formed parts of the land surface by dumping on it rather than carving or plucking away at it. To understand why this is so, we need first to consider what has hap- pened on a continental scale during the last 2 1/4 million years, just the latest short span in the current 65-million-year era that geologists call the Cenozoic (literally meaning the age of recent animal life). Two and a half million years ago marks the beginning of the current ice ages, during which sheets of ice up 16