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Arco
The Big and Little Lost River valleys have seen many visitors who left no traces of their presence. Each of these valleys was a route for fur trapping parties between 1813 and the 1840s. For a number of years after the trappers left, little took place in the area, but after European visitors came again, this time to search for precious metals, some stayed and small farming communities began to form.

A stage line, started by Alexander Toponce, connected the Salmon River mines and Challis with the railroad at Blackfoot. A stage station was established on the Big Lost River to serve this line. It was known as Kennedy Crossing and was about 5 miles south of the present town of Arco. Because the Challis route and another leading to the Wood River joined here, application was made for a post office, to be named Junction. There were too many places named Junction and the postal service did not want another one. The U.S. Post Office suggested the name of Arco, to honor a visiting count, who had never been to Idaho. The citizens needed postal service and accepted the name.

Some have suggested that the name, instead, came from a rancher named Arco Smith, but perhaps he was named for the town instead of the other way around. In 1880, the stage station moved to another site south of the present town and remained there until the Mackay Branch of the Oregon Short Line Railroad Company was built through the area in 1901, at which time the town moved again, to its present location.

Simplified subsurface stratigraphy west of the Idaho Chemical Processing Plant (ICPP) at the INEEL. This reconstruction is based on interpretation of drill-hole data. The subsurface consists of basalt lava flows and thin sediment interbeds, (from McCurry and others, 1994.)

Lost River Range and the Big Lost River Valley
The Lost River Range contains one of the best continuous exposures of Paleozoic sedimentary rocks in Idaho. The rocks of the range are tilted eastward, and the range is bounded on the west by the segmented Lost River normal fault, which was last active in October, 1983, at Borah Peak. The interior of the range is rugged and forbidding. There are few roads and fewer perennial streams, since the porous limestone generally soaks up the snow melt and any summer rain.

The Big Lost River valley remains grand and empty, populated by a few hardy ranchers, miners, and government workers. Mackay, the only town of any size north of Arco, started as a copper mining service town, and attracted a diverse gathering of ethnic Pioneers (Green, 1992). Here the "Idaho Cowboy" remains a mythology and even a viable life-style. Very little of the "Californication" that has so drastically changed the Wood River Valley has yet reached the waters of the Big Lost.

Craters of the Moon
The Craters of the Moon National Monument, established in 1924, contains the products of basaltic volcanic activity between 15,000 and 2,100 years ago. The monument contains superb examples of pahoehoe and aa type basalt lava flows, cinder cones, lava tubes, spatter cones, and tree molds. The area is well studied geologically, and is a showpiece for basaltic volcanic features. References on the geology of the area include Kuntz and others (1987; 1988).

(left) Folded Paleozoic limestones at the south end of the Lost River Range directly east of Arco. View looks northeast. Arco is in the tree-covered area in the center right distance, (June, 1992). (center) Spires of rafted lava and spatter in the North Crater basalt lava flow (Kuntz and others, 1989). Looking north toward Pioneer Mountains, Craters of the Moon National Monument, (May, 1991). (right) The crumbling remains of the mill of the Empire Mine in the White Knob Mountains southwest of Mackay (Oct. 1995). The mine operated continuously from 1902 to 1930 and sporadically after that to the 1960s. It produced over $9,000,000 of metals, mainly copper. It was the base of the early economy of Mackay.