This span is a ~130’ through Pratt mainline railroad bridge of circa 1880, one of at least six that were recycled to minor rail lines in Sonoma and Marin Counties circa 1905-10. A CalTrans bridge report for one of these mentions drawings with the initials “G, H & S A”, suggesting the Galveston, Harrisburg & San Antonio railroad, the Southern Pacific’s subsidiary in Texas. I believe these bridges represent original spans from the SP’s Sunset Route, dating from 1877-83. Similar spans were cascaded down to many of the SP’s branch lines on the West Coast, and to associated private lines like the Diamond & Caldor, in the same time frame, all part of SP’s Harriman era upgrading.
Of these six, one was sold to the Gualala lumber mill and erected on steel cylinder piers at their logging railroad’s crossing of the North fork of the Gualala river. As of 2018 it still stands on those piers, albeit raised about 6’ to clear floods, on the Gualala 501 Rd. On the Northwestern Pacific’s narrow gauge line along Tomales Bay to Duncan Mills and Cazadero, two spans replaced the Howe Trusses at the mouth of Keys Creek in 1906. The steel cylinder piers remain, adjacent to highway 1, two miles south of Tomales. In 1930, as the narrow gauge was lifted, one was re-erectd over Lagunitas Creek and Sir Francis Drake Blvd., in what is now Samuel P. Taylor State Park. That one was long since replaced by a modern bridge for the Cross Marin trail. The remaining three were used on the NWP’s standard gauge Russian River branch, replacing the washed out second crossing of the Russian River near Northwood, above Monte Rio, in 1909. That branch was abandoned in 1935. Several of its bridges were recycled for highway use, the Northwood spans among them. Two were erected on the Stewart Point – Skaggs Spring Road, at the Gualala River and Haupt Creek crossings, and one on the Cloverdale – Geysers Road over Big Sulphur Creek. The Gualala R. span was replaced circa 1995, while the remaining pair are still visible in Google Satellite View as of 2018/03.
These are typical examples of Phoenix patent bridges built by Clark, Reeves & Co. of the Phoenix Bridge Works. The patented feature involves the columns (posts or compression members), wrought iron tubes assembled from segments riveted together along radial flanges. This makes an extremely efficient post, in terms of strength to weight ratio, but one lacking in facility for lateral connections to bracing elements. The Phoenix column was widely used in the 1870’s and 80’s, in both bridges and buildings, but had been superseded by 1900. For further information, see Thomas R. Winpenny’s history of the Phoenix Bridge Co., “Without Fitting, Filing, or Chipping”.