Silver Creek Cliff geology

For obvious reasons, this blog has featured plenty of sedimentary rocks, but hardly a crystalline rock (one notable exception being World of Stone, which also allowed me to title a post after an obscure George Harrison song; no particular reason although in hindsight I might have been feeling down at the time). I just got back from a trip to Duluth and the North Shore and discovered my phone had secretly and unexpectedly created this grand panorama of a spectacular roadcut, so I thought I'd mix up the usual topics:

Definitely worth the click to embiggen

This location is the north end of the Highway 61 tunnel at Silver Creek Cliff, where there is a fun roadside pullout with information about the construction of the tunnel (and the former route of Highway 61, which went where the walking path is now and looks like it would have been extremely narrow). What we're looking at is a sequence of events in the old Midcontinent Rift, 1.1 billion years ago. There is an annotated photo showing part of the roadcut here, but most of the different things are easy to spot without too much guidance. On the right, the somewhat pinkish rocks beginning above the road are flows of andesite, a type of volcanic igneous rock. The andesite is cut off by a stark, irregular contact with a dark blue-gray rock; the contact begins near the blue minivan and rises going to the right. The darker rock is diabase, an igneous rock that intrudes into existing rock underground and has a similar composition to basalt. The diabase makes up most of the roadcut, but you may notice a weird "scar" running through it. It begins, from the perspective of the flat image, just right of the right-most metal structure on our side of the road (the one right of the street light pole) and rises to the right until it is lost under vegetation. This "scar" is laced with light-colored rocks. It represents diabase that has been altered by faulting, with new minerals forming in the fault gouge. Thus, the sequence of events is: 1) eruption of andesite; 2) intrusion by diabase; 3) faulting and mineralization within the diabase.

The key part of the panorama, showing the andesite, diabase, and fault.