Geology on the Road: Slate near Connors Pass, Highway 50

If you drive east of Connors Pass on Highway 50, to the location embedded in the map at the bottom of this post, you will come to this wonderful roadcut exposure of slate, along with many other wonderful roadcut exposures before and after this one (including a few faults, some shattered limestone, and much more slate). Watch for places to pull out while going downhill to the east (there are fewer places to pullout when going uphill to the west), and listen for the heavy truck traffic (and fast cars) when crossing the road. This part of Highway 50 overlaps with U.S. Route 6, an overall busier truck route.
I stopped here to get some garden rocks, but spent a lot of time - at this exposure and at others - checking out the geology. The phyllitic, calcareous slate (to slaty phyllite) is medium to dark gray, usually shiny, and before regional metamorphism it was either a thin-bedded shaly limestone or a limy shale (or both). It was assigned to the Lincoln Peak Formation by Drewes (1967), according to Hose and Blake (1970) - neither map is available online.
For this closer view, the hammer handle is parallel to lineation on the phyllitic slaty surface, and the hammer head is pointing parallel to some weak crenulations, which are stronger above and to the left of the hammer.
I love this shiny rock!
Here, I'm looking edge on at the slaty cleavage and parallel to the crenulations. Folding of the slaty cleavage is weak; there are a few cracks breaking through the slate at some of the crenulations - these may represent a weak or incipient crenulation cleavage. Looking edge on into the crenulations is difficult in the nearby roadcuts up and down the highway because the slate usually dips fairly steeply towards the road. This view requires considerable placing of face next to rock. Little crenulation folds like these can usually be seen only in float pieces.


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