The example of fissile metamorphic slate illustrated here came from along Curve #149 on Track #2 of the Sierra grade where it had fallen from the excavated face of Cape Horn. Originally formed under the sea over 200,000,000 years ago during the Mesozoic era as sedimentary rock, it was eventually lifted up and slowly changed into its present form over millions of years by heat and pressure exerted on it from below by pools of molten magma rising through the earth's crust. When this underlying magma finally cooled it became one of the some 200 granite plutons which now make up the sierra nevada batholith.
The original Cape Horn grade, as located by resident engineer Lewis M. Clement, was a narrow ledge just barely wide enough to support the single track it carried. Constructed in 1865-66 by the CPRR's industrious crews of Chinese workers, it was literally "carved" out of the rock using hand drills and black powder, and by filling in a number of deep gullies between the outcrops. (The SP finally widened the grade to its present contour in 1929.)
Unlike the extremely hard granite underlying the mountain, the surface metamorphic rock is relatively soft and fractures easily. For that reason pieces large and small tend to continuously slough off the wall above the track. This is especially the case along Curve #149 on the West side of Cape Horn where the excavated wall is the highest and steepest.
To help avoid damage to passing trains and prevent potential derailments caused by falling rock, the inside of Curve #149 is protected by an electric rock slide detector fence to warn the crews of approaching trains of debris on the track. To help minimize major delays caused by larger falls, Maintenance of Way crews from the UPRR also periodically trim rock off from above the grade which has been loosened by weathering.
(Click on the images below to see at full size.)
USGS Geology Topographic Maps of Cape Horn – Cape Horn Formation (fissile clay slates)
1892: Economic Geology; Areal Geology; Map Legends
Cape Horn Aerial Photo