Well, another great week to look at rocks! We explored about a five mile stretch of Big Branch, definitely the biggest drainage left unexplored on the Shirley quad at this point. Quite a bit of Cane Hill in the bottom, then several hundred feet of Witts Springs above. The rocks near its confluence with Weaver Creek are dipping strongly southeast, and the Cane Hill actually dives into the subsurface there.
Still haven’t decided if the big structure in Weaver Creek valley is a fault or just a really big monocline, but we’re leaning toward monocline right now because we still haven’t seen a real break in the rocks. Of course, faults typically become covered because the fractured rock is preferentially eroded. It just seems less and less likely that there is a fault there with each drainage we do that should cut across it.
Monday we walked in the lower end and got several strong SE dips in the Cane Hill. Then we discovered an outstanding outcrop of basal Witts Springs sandstone, that we thought was a very large boulder at first because of the advanced state of the sort of “dry weathering” that usually affects the massive Witts Springs boulders after they become separated from groundwater, usually along joints, as they slide downslope. This includes well-developed honeycomb taphoni, well-defined liesegang banding, and case-hardening of the surface. In the bluff face, solutionally-enlarged joints can form fracture caverns, and spalling near the base can form bluff shelters. All of this can happen under the influence of groundwater of course, but that kind of saturation usually leads to a punky or rotten texture in the rock, and forms very steep, covered topography. The really spectacular outcrops occur when lack of groundwater slows down the weathering to a grain-by-grain process. This is what I call “dry weathering”. After walking up both sides and along the top, we concluded that it was indeed part of a continuous outcrop that was probably protected from groundwater penetration by its joint system. I dubbed it “Castle Rock” because of its many turrets and towers.
Speaking of weathering, on Tuesday we saw a classic example of preferential weathering along beds of sandy limestone interbedded with limey sandstone. When these beds are freshly exposed, they form light and dark bands within a smooth face of massive sandstone. We refer to this informally as “zebra rock”. The light bands are more limey, the dark bands less. As weathering progresses, chemical weathering breaks down the more limey areas at an accelerated rate simply because there is more reactive material in that rock than in the sandier beds around it. When weathered, these areas form long horizontal hollows or pits in the massive sandstone.
We informally refer to this weathering pattern as “Prairie Grove weathering” after a Member of the Hale Formation in northwest Arkansas that most typically exemplifies this trait. The base of the Witts Springs Formation is an equivalent unit to the Prairie Grove Member, and often massive sandstones within the Witts Springs will display this type of weathering as well. Though not definitive, this characteristic can be used to help us determine if a given outcrop is within the Witts Springs.
As we made are way up the Big Branch, we ran into our old nemesis: the February 5, 2008 tornado track. This is the one that was on the ground for 122 miles in Arkansas and killed 14 people. We’ve crossed it’s track on several maps, and it never ceases to amaze how destructive it was.
I’ll be back next week. Until then, see you on the outcrop!