This week we finished up a few odds and ends on the Shirley quad. We needed to get to a few suspected outcrops along the north side of the Middle Fork just east of Shirley. As we were looking for a way to access them, we stumbled upon the Sid Burgess Historic M&NA Trail which starts in downtown Shirley and ends up about a mile distant at the historic Cottrell-Wilson Cemetery. As luck would have it, this trail happened to access the very areas we needed to see. If you’re ever in Shirley, it’s definitely worth checking out!
We saw mostly thin-bedded sandstone and shale units of the same variety as on the south side of the Middle Fork and Weaver Creek upstream. There are a few low dips toward the lineation, but nothing indicating a major structure. I’m thinking this may all be the unit above the Witts Springs (Bloyd Formation) brought down to the southeast by a monocline. The trouble is, we don’t really know what the Bloyd/Witts Springs contact looks like in this area yet. That’s something we still need to work out.
Tuesday was wet again, but we set out to finish up the southernmost branch of Lost Creek anyway. Seems to be mostly Witts Springs in there with some Cane Hill at the bottom of the valley. We saw some great examples of soft-sediment deformation in some of the silty units on the way down. Soft sediment deformation occurs during sedimentation when the rapid loading of usually more dense, overlying sediments causes the less dense, buried deposits beneath them to become partially liquefied, which forms various types of disruptions in the original bedding. This can take the form of simple reorientation of the bedding as we have here, to more complex convolute bedding and flame structures. I took a photo later in the week of a good flame structure in the Bloyd Formation. Notice where the shale has been squeezed up between the thick, contorted beds of sandstone.
Several massive calcareous sandstone units in the Witts Springs again illustrated the dramatic difference between outcrops weathered with and without the influence of groundwater. Notice how rotten the outcrop of massive sandstone in the photo below left appears. Also note the green color. There is a layer of moss and lichen growing over almost the entire rock surface, made possible by its relative saturation by groundwater. These organisms help accelerate the weathering of the rock, and there are places where you can actually see clumps of moss peeling off the surface along with a layer of sand. This type of chemical weathering is known as chelation and results in the effective removal of the residual iron cement still holding the rock together after the calcite cement has been dissolved by groundwater. The photo below right shows how “dry weathering” of a boulder of the same material can result in well-defined liesegang bands. Highly concentrated iron has cemented these bands within the massive sandstone, and without the influence of groundwater, they are preferentially resistant to weathering, leaving them in bold relief.
On Friday, we looked at some of the last steep areas we haven’t vistited north of the Middle Fork east of Shirley. Definitely still have Witts Springs right down to the river there, but there is also a thin- to very thick-bedded unit above it that is probably in the Bloyd. We saw a fairly recent landslide above the river composed of material from that upper unit. There was also a good cut and fill channel bed exposed in that unit as well.
It was warm enough for the critters to be out again this week. Just when I thought it was safe to put my foot down anywhere I pleased, I nearly stepped on a moccasin. That’s him slinking back in his hole. We also saw a western slimy salamander (plethodon albagula?) under some storm debris, which was subsequently replaced.
Hopefully the warm weather holds out, but the forecast says the bottom may drop out on Friday. We’ll see!
Until next week, see you on the outcrop!