Why is Boxley Valley so Wide?

Have you noticed how wide the valley is at Boxley Valley? In 'How Rivers Work 101' (intro fluvial geomorphology), we learn that channel and valley width increase downstream along with discharge. A look at a topo map, like the clip from the Boxley map below, or a drive along Boxley Valley to the low water bridge at Ponca where the valley starts to pinch down shows that the Buffalo River is an exception to the rule. For our research team, one of the central scientific research questions is: how does rock type affect river processes? To learn more about valley width, we made measurements of valley width at regular intervals using geographic information systems (GIS) along the river and then partitioned the measurements by lithology to test whether our observation of valley width changing with rock type was statistically relevant. We found that it is. In the limestone reaches (like the reach from Boxley Valley where the Buffalo River begins to the the low water bridge at Ponca) the valley is 70% wider on average than in reaches that are more sandstone dominated (like the reach from Ponca to just upstream of Carver). While it is evident that lithology and valley width are related in the Buffalo River, we're still working to discover how rock type effects the erosion processes that control and produce the variations in valley width.  You can download the full version of the maps shown below from our Maps Page and check back for more research results from our team!   

The geology map at Boxley Valley shows the wide valley where the channel is incised into the Boone Formation limestone. Excerpt from Hudson, M.R., and Turner, K.J., 2007,  Geologic map of the Boxley Quadrangle, Newton and Madison Counties, Arkansas : U.S. Geological Survey Scientific Investigations Map 2991, 1:24,000 scale.

The geology map at Boxley Valley shows the wide valley where the channel is incised into the Boone Formation limestone. Excerpt from Hudson, M.R., and Turner, K.J., 2007, Geologic map of the Boxley Quadrangle, Newton and Madison Counties, Arkansas: U.S. Geological Survey Scientific Investigations Map 2991, 1:24,000 scale.

The geology map from Ponca low water bridge to Steel Creek shows that where the channel cuts into the Everton Formation, the valley becomes narrower. Excerpt from Hudson, M.R., and Murray, K.E., 2003,  Geologic map of the Ponca Quadrangle , Newton, Boone, and Carroll Counties, Arkansas: U.S. Geological Survey Miscellaneous Field Studies Map 2412, 1:24,000 scale.

The geology map from Ponca low water bridge to Steel Creek shows that where the channel cuts into the Everton Formation, the valley becomes narrower. Excerpt from Hudson, M.R., and Murray, K.E., 2003, Geologic map of the Ponca Quadrangle, Newton, Boone, and Carroll Counties, Arkansas: U.S. Geological Survey Miscellaneous Field Studies Map 2412, 1:24,000 scale.

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.

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!

Welch Bluff

Lat/Long: 36.064716, -93.126730

This is a view looking east across the Buffalo River at Welch Bluff showing the normal fault zone that forms the southern margin of the Braden Mountain graben. On the hanging wall (on the left side of the image to the north), the contact between the Boone and Everton Formations is dropped below the river level. On the footwall (on the right side of the image to the south), the contact rises above river level. The graben system formed during the late Paleozoic (~250 Million years ago) in association with the Ouchita uplift. The fault is fairly quiet now. If it was active, we might see a change in the gradient of the Buffalo River here, but it is constant here.  The info about the fault was adapted from Hudson et al., 2011 and Hudson 2000. So check out those papers for more scientific detail!  For more information on how faults work, see the USGS page on faults.

Photo of Welch Bluff showing the offset of the geologic stratigraphic formations.

Photo of Welch Bluff showing the offset of the geologic stratigraphic formations.