Pedestals are a fairly common erosional feature in Arkansas in places where conditions are favorable. They typically form in massive sandstone units due to an increased rate of erosion along the joint set near a bluff line. Joints are vertical fractures within almost all rocks that formed in response to the tectonic stresses they have undergone in the distant past. Joints are most often expressed as sets oriented in rhombohedral patterns. Water can more easily penetrate the rocks along these joints, eventually opening a gap. When this happens along joints parallel to a bluff face, the gap essentially cuts off the incipient pedestal from the influence of groundwater, isolating it from most of the processes of chemical weathering. Once that happens the majority of weathering of the newly formed block of sandstone is done by wind and rain. Because the corners and edges of a rhombohedron have more surface area, weathering is concentrated there, eventually rounding it off to form the typical pedestal shape. In many places, a capstone of more resistant sandstone is present which contributes to the top-heavy pedestal or mushroom shape. Also, the pedestal-forming unit is commonly underlain by shale or silty-shale on which the fully intact pedestal can slowly creep downslope. Some of them end up quite a distance from the bluff where they started. If you would like to view several fine examples of this erosional phenomenon, consider a visit to Pedestal Rocks Natural Area in the Ozark National Forest.
Taphoni (honeycomb weathering) in massive sandstone.
Sorry about that long hiatus, but I had a couple of extra projects the last couple months that took a lot of extra time. We’ve been in the field almost every week except for March 3-5 during the 3 inch snow in Van Buren County. We’ve mostly worked on the Fairfield Bay quad during the last few weeks. This week was spent tracing a very thick-bedded, massive sandstone unit through the town of Fairfield Bay itself. It is quite an impressive bluff-former and actually underlies almost the entire Mountain Ranch golf course.
Danny descending treacherous massive sandstone outcrop
Danny contemplating how this massive sandstone can all but disappear a few hundred yards north of here
Grotto in massive sandstone
Most hillsides are composed of a thick sequence of very thin sandstone/siltstone and shale–easily erodible
Apparently some structure or perhaps a change in depositional environment made this sandstone climb up 200 feet to the east. There it forms the cap of the ridge on which the small town of Fairfield Bay sits. Moving east again, It underlies the Indian Hills Country Club where weathering (and earth-moving equipment) has produced the famous Indian Rock House on the golf course there. Underlying that massive across the entire area is a very thick sequence of very thin-bedded sandstone/siltstone/shale. A lot of the roads built in this unit have formed deep gullies making some of them impassable. Still, there is better access in this area than most that we map, so we’re thankful for that. Only about two weeks left of the field season. We’ll probably be jumping around a lot to work out problem areas on both quads during that time.
See you on the outcrop!
Danny actually seeing through the groundcover to the rock beneath the Mountain Ranch golf course