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.


What is that round thing in the rock?

Kevin Middleton sent us a message with this photo and asked about the round feature in the top right of the cliff. Here's what our expert research geologist, Mark Hudson (USGS), had to say about it:

From the overhang and the knobby nature of the outcrop I would guess that this is a middle Bloyd sandstone cliff. In that context (i.e., porous sandstone) it is most likely that circular patterns are iron concentrations (Liesegang bands) formed from groundwater seepage that make the bands more resistant to weathering and stick out. I don't commonly see them quite this circular, so this is an interesting example. Of course one would like to get a closer look to verify the guess.

Send us your geoscience questions we'll do our best to answer! Thanks for sharing this with us, Kevin!