Work on the Bagley Tunnel began in 1877, but proceeded sporadically until 1904, when development of the tunnel accelerated until it reached its final length of 7,500’ in 1911. The tunnel is notable for its size, symmetry, absolutely straight course through Houghton Mountain, and lack of timbering due to the strength of the surrounding rock. The Bagley Tunnel did not follow a specific vein but was driven to cut major mineral veins at a perpendicular angle and may be one of the earliest examples of crosscut tunneling in northern San Juan County.

Constructed in 1912, the Frisco-Bagley Mill is a rare example of a prefabricated early 1900s concentration mill in northern San Juan County. The mill’s structural components were pre-cut, pre-fit, and number/letter coded in Silverton and then transported to the site for assembly. In addition to its distinctive method of construction, the mill represents an innovative adaptation of the standard mill design. To reduce operation costs, the majority of the milling equipment was installed on a single level and sheltered by a gable-roof superstructure. Concentration mills that retain a significant portion of their superstructure are rare in northern San Juan County and the Frisco-Bagley is the only known example of this design variation. 

The Frisco-Bagley Mill and Tunnel complex is listed under the Historic Mining Resources of San Juan County Multiple Property Submission.