Monthly Archives: April 2014

Geopic of the Week

 

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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.       

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Statemap Field Blog March 24-26, 2014

Taphoni (honeycomb weathering) in massive sandstone.

Taphoni (honeycomb weathering) in massive sandstone.

Hello all!

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.

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Danny descending treacherous massive sandstone outcrop

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Danny contemplating how this massive sandstone can all but disappear a few hundred yards north of here

 

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Grotto in massive sandstone

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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!

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Danny actually seeing through the groundcover to the rock beneath the Mountain Ranch golf course

 

 

 

 

 

Statemap Field Blog, Dec 16-18, 2013

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Hello all!

Well, it’s been a week and a half since the snow came down, and there are plenty of shady areas where it’s still on the ground.  We started out on the east side of the map in Deadland Hollow, even though it was north-facing, because we knew we could get to it fairly easily.  If anything, the snowy areas may even have been a little more treacherous this week, because it’s thawed and frozen so many times that it’s more like solid ice now.  After that we went all the way over to the west side of the map and got a few points in a small drainage south of Weaver Creek.

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On Tuesday we walked from the lake to the top of Dave Creek in an undeveloped part of Fairfield Bay.  Saw some really good worm burrows in a thin- to medium-bedded sandstone  near where the creek reaches the lake.  Some of these burrows crossed bedding planes, so the rate of sedimentation must have been fairly rapid during deposition of this unit.  2013-12-17 0102013-12-17 030

Above that, we were in fairly continuous outcrops of calcareous sandstone, including some beds of “zebra rock” (see previous two posts).  We can only surmise that we are again in the Witts Springs Formation, which is interesting in that it’s still at the surface south of that Weaver Creek/Middle Fork lineation.  This probably means that if the lineation formed due to a fault at the surface, there is minimal offset.  More likely, it indicates the lineation formed along a monocline at the surface perhaps indicating a fault at depth in the basement rock.  Anyway, we had several hundred feet of calcareous sandstone along the creek, some of which exhibited large scale cross-bedding.

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We’re still working on that Bloyd/Witts Springs contact, but probably did cross it somewhere in that upper end.  There may even be some Atoka up there, but if so, it will be in sparse exposures at the very highest elevations.  If the Bloyd proves to be several hundred feet thick here as it has been on other quads to the west, the only Atoka may be on the southern third of the Fairfield Bay quad south of the southwest/northeast lineation that goes through the upper end of Greers Ferry Lake.  This is almost undoubtedly a fault at the surface based on the steep dips we’ve been seeing on the north edge of the lake.

On Wednesday, we started at the lake again and went up a small drainage on the eastern edge of the map.  Had a great view of Sugar Loaf Mountain when we started that morning.

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Well, I hate to say it, but this will be the final blog of this field season.  I have a major test and a GSA field trip to prepare for this spring, so I need to devote all my time to that right now.  It’s been fun to write it, and I hope I’ve given you a better idea of what range of effort goes into making a “simple” geologic map.  We’ll keep going out until mid-April, then we’ll have about 10 weeks in the office to draw and digitize the two maps, add descriptions of the rock units, a cross section, stratigraphic column, joint diagram, and correlation of map units.  If all goes according to plan, we’ll turn the finished maps into the USGS on June 30 to fulfill our grant requirements.  The Shirley and the Fairfield Bay quads should be up on our website as a .pdf by mid-July.  By that time, we’ll be back in the field battling ticks and snakes next season–probably on the Parma and Greers Ferry quads.  Who knows, maybe I’ll start blogging again!  Until then, see you on the outcrop!

Statemap Field Blog, Dec 10-12, 2013

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Hello all!

Well, we did have a winter weather event last Friday that was still on the ground in the field area the following Tuesday.  Roads were fairly clear to Clinton, but between Clinton and Shirley it was still mostly patches of snow and ice.  North of Shirley, the roads were pretty much all covered with snow and ice.  Even “Goldy” (our Jeep) got stuck briefly when Danny decided to stop and check his map almost at the top of a hill.

Tuesday we managed to get to a drainage south of the Middle Fork that had fairly low relief, so were able to climb down the side and follow it on down.  There were some very large footprints in the snow along the valley floor that may have been a feral pig, but don’t know for sure.  It’s amazing how the critters always choose the easiest route.  Their trails are usually pretty good for people too, though they don’t often care about avoiding briar patches.  The snow was pretty crunchy, so fairly good traction.  Having snow on the ground rather evens out the terrain in an odd way, though you have to be poised to catch yourself with every step.  Had some massive sandstone units, but they were blocky and non-calcareous, so we’re in something different than on the north side of the river.

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The next day we managed to make our way north of Shirley to a couple drainages at the head of Indian Creek that we had skipped when we did the lower end.  As it happened it was a pretty good choice because the relief was rather low and the bottom was fairly wide.  Also, it was south-facing which helped to melt the crusty snow and maybe keep us a little warmer.  We saw mostly massive calcareous sandstone units of the Witts Springs though may have gotten into the Bloyd in the higher elevations.  At the end of the day we entered an area that still showed signs of damage from the Jan. 2009 ice storm that coated most of northwest Arkansas with a thick layer of ice and downed many trees–many by the roots.  Witnessed a beautiful sunset on the way out though.  We were near 1400 feet which is about the highest elevation on the quad.  The trees there were still coated in ice from last Friday’s storm.

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Thursday we tried another drainage near the one we hiked on Tuesday.  This one was north-facing and proved to be too steep and narrow a descent to do with so much snow on the ground.  The snow had developed a thick icy crust from thawing and refreezing, and we had to break through it to get traction.  I managed to get up and around on the side of a particularly narrow spot in the creek bed, and was waiting for Danny to follow when I saw him retreating back upstream.  I later found out that he couldn’t find purchase on the icy banks, so decided to return the way we had come.  Since I had gotten farther downstream by breaking through the crusty snow and climbing down the steep side, I had no choice but to climb back up that way.  I found that going on up was less hazardous than going back down toward the creek, so I continued to climb up the side and by the time I drew even with Danny was probably 100 feet higher.  I could barely see him down there, but managed to get a photo of him climbing up the drainage.  We’ll have to try that one again when the snow is gone!

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After that, we got a few points on Weaver Creek were it leaves the western edge of the map, and headed back to Little Rock.  Hope this clears up by next week!  Until then, see you on the ice-covered outcrop!

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