calc-alkalic to tholeiitic to basaltic andesite lava. Goats Rocks and its immediate vicinity are underlain by pre-Tertiary greywacke and argillite, and these deposits are cut by several prominent northward-trending faults. Overlaying these deposits are Cenozoic volcanic rocks, including the thick, Eocene-aged Ohanapecosh Formation, comprised by chemically altered basalt and andesite lava flows and bedded andesite and dacite volcaniclastic rock. In turn, the Stevens Ridge Formation overlies the Ohanapecosh layer, featuring quartz-bearing silicic tuff, rhyolite, and other volcaniclastic rocks. Under both layers is the Russell Ranch Formation, to in thickness, which contains deposits of lapilli and tuff hinting at a caldera, although geologists have failed to identify clear evidence for the formation of a caldera in this zone. Together with Mount Adams, Mount Rainier, and Mount St. Helens, the Goat Rocks volcano is part of a triangle of volcanoes, an arrangement not found elsewhere in the Cascades. This may be related to a mid-crust zone with abnormally high electrical conductivity, the Southern Washington Cascades Conductor (SWCC), which is not well understood by geologists. Scientists from the United States Geodynamics Committee hypothesized in 1994 that this anomaly is associated with the thrust of a large deposit of sedimentary rock against a continental margin. However, Hill et al. (2009) dismiss this view as it proves inconsistent with maps made from magnetotelluric readings of the area. Instead, they propose that the anomaly is linked to the region's volcanism, which has caused partial melting of the crust. The oldest Cascade stratovolcano formed after the Columbia River Basalt Group, Goat Rocks is roughly equal in volume to Mount St. Helens and smaller in volume than Mount Baker, at approximately . Rough estimates place the composition of the volcano as 40% tuff and breccia. The remains of its lava flows can be found in the Tieton, Klickitat, and Cispus river valleys, their elevation reversed by erosion to make them resemble ridges. These flows have been thoroughly eroded, but testify to the volcano's previous size. Glaciation Although it may have once been a towering stratovolcano, Goat Rocks has undergone extensive erosion since its extinction. Once eruptions ceased, the ongoing forces of glacial erosion, as well as hydrothermal alteration of the volcano's interior, stripped away layers of volcanic ash deposits and lava flows, uncovering the rocky lava spines where magma had cooled and hardened within the conduits of the volcano. Nearly all the remaining volcanic debris has been partially