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.

Fossil Canids of the National Park Service

We've been taking a tour of Mammalia for the past two annual "Fossil [Group] of the National Park Service", and this year we'll make it three in a row with man's best friends, the Canidae. The dog record starts up in the Eocene with the hesperocyonines, who held court through the Oligocene but then petered out in the Miocene. Turning up in the Oligocene are the borophagines, or "bone-crushing dogs," and the canines, which include wolves, foxes, domestic dogs, and close relatives. Borophagines, as the nickname suggests, had robust jaws and teeth, which doesn't mean they should be typecast as slavering hypercarnivorous brutes (canids in general have been pretty flexible about diet over their history). They drop out of the record at about the end of the Pliocene, leaving the field clear for the canines, which had been a fairly minor component of the canid radiation until about the Late Miocene. If you're interested in paleontological nitty-gritty on these two groups, check out Wang et al. (1999) on borophagines and Tedford et al. (2009) on canines. Canids, incidentally, are a North American invention, and unlike some other groups that originated in North America (camels, horses, rhinos), they have never gone extinct on the continent.