The Middle Bloyd Sandstone

Middle Bloyd Sandstone

The middle Bloyd sandstone is not yet a formally recognized unit, but these are some of the most distinctive rock units in the Buffalo River region. The middle Bloyd is considered part of the larger Upper Bloyd Formation that is a sequence of sandstone with some interbedded siltstone, shale, and limestone. The middle Bloyd. It is a Lower Pennsylvanian (~299-307 Ma), Morrowan sequence of sandstone that contains quartz pebbles, and lycopod fossils (Hudson, et al. 2001) that is 80-120 ft (~24-37 m) thick. You can see it outcropping in many of the bluffs in the highest parts of the watershed. The middle Bloyd makes up some well-known bluff formations including Buzzards Roost in northwestern Pope County and the outcrop at Horseshoe Canyon Ranch where you can rock climb on and get a close-up look at these rocks. This photo is taken from AR state Hwy 43 at Mt. Gaither between Ponca and Harrison (see our Geosites page for location). The middle Bloyd is present in the Boston Mountains Plateau sub-province of the larger Ozarks Plateaus physiographic province. The middle Bloyd is recognizable because of the distinctive cross-bedding (e.g. Unrein 2007; Hudson et al. 2011). The figure below from Kevin Unrein's master's thesis describes some of the cross-bedding of the middle Bloyd at Mt. Gaither. The middle Bloyd was first differentiated from the rest of the unit by Zachry 1977. There is still a lot of scientific research interest in the middle Bloyd. The current explanation is that the formation was deposited as during a transition from a fluvial to estuarine system in the Morrowan stage of the early Pennsylvanian subperiod of the Carboniferous Period (e.g. Unrein, 2007). You can find more information in Kevin Unrein's thesis (downloadable on our Scientific Papers page), Mark Hudson's maps (on our Maps page), and in the Geological Society of America Karst Interest Field Trip Guide (downloadable on the Guides page).  You can find the location of this photo on our Geosites page

Description of cross-bedding of the middle Bloyd sandstone at Mt. Gaither from Unrein, 2007. (See full citation on our  Scientific Papers page .

Description of cross-bedding of the middle Bloyd sandstone at Mt. Gaither from Unrein, 2007. (See full citation on our Scientific Papers page.

       

Roark Bluff geology

Roark Bluff upstream end

This is the upstream end of Roark Bluff at the beautiful deep swimming hole across from the campground. The dashed line indicating the boundary between the Newton Member of the Everton Formation and the lower part of the Everton Formation is approximate. Mark Hudson, one of the geologists on our team, has sampled tufa at the base of this cliff, and has verified that it is the lower Everton Formation (Oel on the geology maps). At the base of the Newton Sandstone Member (Oen), there is a change to more massive, rounded bedding higher up on the cliff. The exact contact is an estimate in the graphic. The differences visible in the photo are more massive beds and rounded texture in the Newton. In the lower Everton the beds are less massive, thinner, and are more angular.

For most, the overhanging ledge that marks the lower quarter of the bluff stands out. That is an erosional feature and its location is more likely related to hydrology instead of stratigraphy. During floods, the rocks below are eroded, undercutting the overhanging rocks which then become unstable and under the forces of gravity detach via rockfall.

Check out our Geosites Map for the location of this site.

Geology of the bluffs at Steel Creek

Steel Creek from Buffalo River Trail

There are a couple of bluffs at Steel Creek. The prominent one across from the campground on the left side of this photo upstream of the boat launch on river left is called Roark Bluff (there is a closer photo is in the previous post). The bluff labelled Bee Bluff on USGS maps is downstream of the boat launch on river right and the right side of this photo. (This was a mistake in labeling the bluffs.  Historically, this bluff is unnamed.) Roark Bluff is made up of the Everton sandstone, a Middle Ordovician interbedded dolostone, limestone, and sandstone. At Roark Bluff, the Lower Everton is present at the base, but the majority of the bluff is made up of the Newton Member of the Everton Formation. The Newton Member is composed of fine well-sorted, very rounded quartz grains and is really thick in the western end of the Buffalo River. This rock has a sugary texture and the sand grains within it are round like little marbles. The channel may incise down to the Powell Dolomite at the base of Rushing Pool (the deep pool across from the campground) but, it doesn't really show up in Roark Bluff and it is unclear if the channel really reaches down to the Powell in this reach. The Powell does appear at river level just downstream of downstream of Bee Bluff on river left where Cliff Hollow incises into it making a nice exposure. The Powell appears at channel level for just under 2 mi from Cliff Hollow to Beech Creek which enters river right. The Boone Formation, including the St. Joe Member do cap the bluffs in this reach but I don't think they are actually visible in the vertical bluff faces at Steel Creek.

I'm working on labeling photos of the prominent bluffs with the geologic units, so stay tuned for more!

Check out the Geosites Map for the location of this view.