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.