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Map of Marsh Valley.
Section 4, Chapter 16 -Marsh Valley
Chapter 16:
Geography and History of Drainage
Henry O. Harkness and early McCammon
Camp Downey
Southern Bannock County

Geography and History of Drainage
Marsh Valley, bordered on the east by the Portneuf Range and on the west by the Bannock Range, is the primary access to the Snake River Plain from the south. As the area was, after 1867, part of the Fort Hall Indian Reservation, legitimate settlement of the southern part was delayed until after 1889 when the land south of McCammon was excised from the reservation. Euro-American settlement of northern Marsh Valley began with the Land Run of 1902.

The Portneuf Range contains east-dipping Lower Paleozoic and Late Proterozoic rocks and the Bannock Range contains the same sequence, repeated across a normal fault that bounds the east side of Marsh Valley. The Basalt of Portneuf Valley is at the surface at the north end of Marsh Valley, having been erupted from the Bancroft area about 600,000 years ago.

The north end of Marsh Valley today contains an example of "inverted topography" with the lava flows filling the middle of the valley and the rivers confined to the sides. The Portneuf River is on the east and Marsh Creek, the main path of the Lake Bonneville Flood, is on the west. At the time of eruption of the lava flow, the center of the valley was lowest, and thus the term "inverted topography."

Geophysical surveys indicate that there is as much as 10,000 feet of valley fill underneath Downey, and that over the last few million years, drainage in Marsh Valley was primarily to the south, perhaps into a lake basin which periodically had an outlet to the Bonneville Basin. The prominent pediment surfaces seen below Scout Mountain and Mount Bonneville were established during this time of southward drainage and are graded to a base level several hundred feet above the present valley floor.

After establishment of the throughgoing drainage of the ancestral Bear River, the base level fell and the pediments were incised. Garden Creek which had been flowing east on deposits of gravel and sand began cutting down and encountered the bedrock ridge at Garden Creek Gap. It continued downcutting, resulting in the superposed canyon seen today.

Osborne Russell's thoughts on leaving southeastern Idaho for the Willamette Valley of Oregon in 1842.

"In July 1842, I ascended to the top of Ross Mountain (probably Mt. Bonneville in the Portneuf Range) on which the snows remain till the latter part of August. I sat down under a pine and took a last farewell view of a country over which I had traveled so often under such a variety of circumstances. The recollections of the past connected with the scenery now spread out before me put me somewhat in a poetical humor, and for the first time I attempted to frame my thoughts into rhyme.

I n the year 1836 large bands of buffalo could be seen in almost every little valley on the small branches of the Portneuf River. But now the only traces which could be seen were the scattered bones of those that had been killed. Their trails which had been made in former years deeply indented in the earth were overgrown with grass and weeds. The trappers often remarked to each other as they rode over these lonely plains that it was time for the white man to leave the mountains as beaver and game had nearly disappeared." (Haines, 1965, p. 123-125).