BLASTING THROUGH THE TACONIC UNCONFORMITY: HISTORY, GEOLOGY, AND ENGINEERING PROBLEMS OF 19TH- AND EARLY 20TH-CENTURY RAILROAD CUTS AND TUNNELS THROUGH SHAWANGUNK MOUNTAIN IN SOUTHEASTERN NEW YORK

In SE New York Shawangunk Mountain forms a 44-mi-long barrier between the Neversink-Rondout Valley on the W and the Hudson Valley on the E. Although the promoters of the Delaware & Hudson Canal proposed a rock tunnel through the ridge as early as 1825 in order to shorten the route to New York City, it wasn’t until 1847-48 when the New York & Erie RR (later the Erie) blasted a deep cut through the mountain at Otisville that the barrier was finally breached. In 1871 the Oswego Midland (later the Ontario & Western) RR completed a 3,857 ft-tunnel (now abandoned) through the mountain between High View and Wurtsboro, 8 mi to the NE. In 1908, the Erie abandoned the original route at Otisville for one of lower grade and completed a second Shawangunk tunnel (5,314 ft long and still active) just N of the old cut.

Shawangunk Mountain is underlain along its crest and NW slope by the Silurian Shawangunk quartzite and conglomerate (Ss, ~ 600 ft thick) and on the SE slope by the Ordovician Martinsburg shale and greywacke (Om, ~10,000 ft thick), the two formations being separated by the Taconic unconformity. The unconformity is well exposed at several localities along the mountain between Otisville and Wurtsboro, most notably in the Otisville RR cut and in a cut along NY 17 above the High View Tunnel. These two cuts provide clues to the nature of the unconformity in the tunnels. The dip discordance is 8^o at Otisville—where a puzzling diamictite (colluvium?) occurs—and ranges up to 15^o at NY 17. At the portals of the High View Tunnel, bedding in the Ss at the W portal is 213/26 and in the Om at the E portal 215-245/10-15. A more complex situation exists at the Otisville Tunnel where bedding in the Ss at the W portal is 227/45, and the Om at the east-portal cut is folded into a W-verging recumbent fold.

Both tunnels have been plagued with maintenance problems, particularly those related to water. Bearing testimony to those at High View are the large pool of water at the E portal and the water flowing from a similar pool at the W portal. Early in construction of the Otisville Tunnel, it was reported that it had large amounts of water seepage into it, causing farmers’ wells to run dry.

Although the High View Tunnel was abandoned in 1957, it is still intact and accessible at both portals. Norfolk Southern and the Metro-North commuter line currently utilize the Otisville Tunnel.