Mardi Krawl 2015: Epilogue
I want to add one last comment, which addresses my original objective in this wheeling/geology trip; why is Mountainside, which is a low-gradient road cut down the north side of Thompson Hollow, more difficult to traverse when it rains than Big Hill, which goes right up the mountain?
My limited field work suggests that the answer lies in the original distribution (horizontally as well as vertically) of clay sediments in the Mississippian. The transition from shallow ocean (i.e. fossil-rich thin beds of limestone and dark shales) to coastal sediments (i.e. the Hartselle Fm. red sandstones and shales with some limestone) during this period produced rapid changes horizontally in facies, which are environmentally dependent groups of sediments (later rocks). I think that coincidentally, there was a barrier island system on the south side of Thompson Draw with thinner layers of mud between thin sand layers. Thus, Big Hill and Mountainside must traverse more shale (now mud) than 1/2 mile to the south. When Mountainside was cut with a dozer, it produced a trail/road that has a greater extent of slippery mud when it rains. This same section (from ~500 -700 feet) on Big Hill is crossed in a series of swales and sandy ridges that are easier to traverse in sections.
There are slippery sections of John’s Way (the escape route out of Thompson Draw) but overall it is sandier.