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Union Pacific timetable, January 27, 1964. Note the reference to "columnar basalt" at McCammon. Click on image for a larger view

The Lake Bonneville Flood
About 14,500 radiocarbon years ago, Lake Bonneville, which occupied much of the presently settled part of Utah, overflowed through a dam of alluvial fan material at Red Rock Pass at the north end of Cache Valley and produced the catastrophic Lake Bonneville Flood which scoured and cleaned loose rocks from the canyon west of Inkom. It is estimated that at Portneuf Narrows the floodwaters were 300 feet deep, (O'Conner, 1990).

The flood removed the Basalt of Portneuf Valley from the Portneuf Narrows area and deposited basalt Boulders which are common in parts of downtown Pocatello. The topography left behind by the flood is called scabland topography, and is manifested in dry waterfalls, alcoves, scoured bedrock surfaces, and Boulder bar accumulations along the flood path. Such topography is easy to see both south and west of Inkom.

The Abandoned Utah & Northern Railway Grade
The Utah & Northern narrow-gauge rail line followed a grade along Marsh Creek and generally on the south side of the Portneuf River through the canyon west of Inkom. It crossed the Portneuf River on a bridge that still exists near Blackrock and ran parallel to the present Union Pacific right of way through Portneuf Narrows.

(left) The streamlined City of Portland, late spring, about 1950. Picture was taken from lava flow just west of Inkom, looking southwest toward Portneuf Narrows. The diesel engines were manufactured by EMD (Electro-Motive Division of General Motors), and were geared for passenger operation to speeds of 105 m.p.h. Bannock County Historical Society Collection. (right) View looking west across Marsh Valley at Old Tom Mountain (left) and Scout Mountain (right). The Basalt of Portneuf Valley fills the low part of Marsh Valley, (July, 1990).