Tag Archives: landslide

Geopic of the week: Landslides

landslide geopic

Here’s a picture of a recent landslide that took out a gravel road south of Greers Ferry Lake in north central Arkansas.  Landslides are one of the natural phenomenon that earth scientists refer to as geohazards.  It’s impossible to predict where and when a landslide will occur, but there are known conditions that make certain landscapes more prone to sliding.

In Arkansas, conditions that can lead to landslides include steep slopes, and poorly cohesive soil or bedrock – such as shale or alluvium.  Land where vegetation has been cleared is also more likely to fail.  Many landslides occur after periods of prolonged heavy rainfall, though that’s a factor that can’t be avoided.  One of the best ways to determine if an area is prone to landslides is to look for evidence of past slides;  If slopes have failed in the past, it’s likely they will fail again.

If you are developing property or are looking at property to purchase, you should consider whether it is in a landslide prone area.   You can always contact a friendly geologist at the Arkansas Geological Survey and ask them their opinion.

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Geopic of the week: The Maumelle Chaotic Zone

Chaotic zone

This is a picture of sandstone and shale of the Maumelle chaotic zone that outcrops along highway 10 west of North Little Rock, Arkansas.  The Maumelle chaotic zone is part of the Jackfork Formation which forms the bedrock around much of the Little Rock area.  The chaotic zone is called that because of the disarray the rock is in there.  In the example above, broken blocks of sandstone are interspersed with disorganized shale beds that have been rolled, squashed and otherwise deformed (rock hammer at center is for scale).  The rocks weren’t deposited this way but were originally organized into horizontal beds on a deep-water ocean slope.  Before they could be hardened into solid rock, the slope failed and the beds were transported down hill in a massive submarine landslide. 

 

Note:  Other interpretations for this zone have been proffered.  The author of this blog prefers the above interpretation.

 

For more views of the Maumelle chaotic zone click here

Statemap Field Blog, March 31-April 2, 2014

 

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Hello all!

Another great week in the field.  Signs of spring are everywhere, and unfortunately the field season is drawing to a close.  We skipped around all over the Fairfield Bay quad this week, still trying to trace the very thick-, massive-bedded sandstone that we’re calling the base of the Bloyd for now.  Just off the eastern edge of the Fairfield Bay quad is a locally famous outcrop of that sandstone that was supposedly visited by Hernando Desoto himself in 1542.  Whether or not that’s true, it is a very impressive bluff shelter known as the Indian Rock House.  A lot of eroded material was removed from the floor of the shelter when the adjacent Indian Hills Golf Club was built, leaving behind the fine sandstone amphitheater we see today.   One could see how this formation could later become a natural bridge if erosion continues along the joint set parallel to the bluff face.  If that interior arch were to fall out, then the remaining one would form a bridge.  This is how most of the sandstone natural bridges in Arkansas are formed.  Lots of graffiti has been scratched into the friable rock over the years, including some that may have been carved by native people.

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On Tuesday, we finished up our field work on the lake.  We still had a couple islands we needed to visit, and the entire south side of the lake is so steep that access by land would be difficult.  We were excited to find more old river terraces on the islands, including one that would have been deposited on a cut-off meander in the area of Harpers Cove.  The deposit is about 80 feet above and over a half mile north of the current river channel (before the lake was there, that is).  The high end of the range for the downcutting rate for the Colorado River in the western Grand Canyon is 16 centimeters/1000 years, and I think we can all agree that downcutting there probably exceeds that in Arkansas.  Using that rate, an estimated 152,000 years would have passed since that terrace was deposited.  That gravel has been there a long time!  Of course, cutting off the meander would have stranded that deposit at that time, but don’t forget that this stream is developed in bedrock, so meander cut-off would be a fairly infrequent event.  To get a better estimate of these events, methods such as luminescence dating are being developed to age date the sand in these stranded river terraces.  With this new technology, perhaps someday we will know when these terraces were deposited.

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On the south side of lake below Stevens Point is a good example of a modern landslide, and a bit of a cautionary tale.  Sometimes clearing trees for roads and houses can have catastrophic results.  The photo tells the story.  The major part of this landslide occurred March 28, 2005 just after a road was cleared from the house down to the lake.  Most of the material at the edge of the lake on the north side of Hunter Mountain is there as a result of old landslides, therefore any development in this area can cause it to become unstable, as evidenced here.  That’s why part of our project includes mapping areas where landslides have occurred.

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Speaking of Hunter Mountain, we ran across one of the now ubiquitous gas well pads up there, and I thought you might be interested to know the function of each piece of typical well head production equipment.  At each wellhead is a set of valves that regulate the flow of gas.  These are often controlled remotely, thus the solar panels which power the system.  The big tanks near them contain hydrogen sulfide which is introduced into the gas right away to give it a strong odor.  This odor is, of course, quite useful to determine if there are any gas leaks since natural gas is odorless.  From the wellhead, the gas flows to the separators which remove any fluids contained in the gas.  This fluid could include heavy hydrocarbons, but is mostly produced water.  These fluids are stored in large tanks which are built inside a berm.  The berm is designed to hold 1 1/2 times the capacity of one of the storage tanks in case of a spill.  The level in the tanks is also monitored remotely and emptied on a regular basis.  From here, the gas is piped to a compression station where it undergoes further treatment.   Then it is sent through a transmission line and on to your house.  It’s not pretty, but for now, we have to have it.

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Well, next week will be the last of our field season.

Until then, see you on the outcrop!

 

Statemap Field Blog—Dec. 2-4, 2013

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Hello all!

This week we finished up a few odds and ends on the Shirley quad.  We needed to get to a few suspected outcrops along the north side of the Middle Fork just east of Shirley.  As we were looking for a way to access them, we stumbled upon the Sid Burgess Historic M&NA Trail which starts in downtown Shirley and ends up about a mile distant at the historic Cottrell-Wilson Cemetery.  As luck would have it, this trail happened to access the very areas we needed to see.  If you’re ever in Shirley, it’s definitely worth checking out!

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We saw mostly thin-bedded sandstone and shale units of the same variety as on the south side of the Middle Fork and Weaver Creek upstream.  There are a few low dips toward the lineation, but nothing indicating a major structure.  I’m thinking this may all be the unit above the Witts Springs (Bloyd Formation) brought down to the southeast by a monocline.  The trouble is, we don’t really know what the Bloyd/Witts Springs contact looks like in this area yet.  That’s something we still need to work out.

Tuesday was wet again, but we set out to finish up the southernmost branch of Lost Creek anyway.  Seems to be mostly Witts Springs in there with some Cane Hill at the bottom of the valley.  We saw some great examples of soft-sediment deformation in some of the silty units on the way down.  Soft sediment deformation occurs during sedimentation when the rapid loading of usually more dense, overlying sediments causes the less dense, buried deposits beneath them to become partially liquefied, which forms various types of disruptions in the original bedding.  This can take the form of simple reorientation of the bedding as we have here, to more complex convolute bedding and flame structures.  I took a photo later in the week of a good flame structure in the Bloyd Formation.  Notice where the shale has been squeezed up between the thick, contorted beds of sandstone.

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Several massive calcareous sandstone units in the Witts Springs again illustrated the dramatic difference between outcrops weathered with and without the influence of groundwater.  Notice how rotten the outcrop of massive sandstone in the photo below left appears.  Also note the green color.  There is a layer of moss and lichen growing over almost the entire rock surface, made possible by its relative saturation by groundwater.  These organisms help accelerate the weathering of the rock, and there are places where you can actually see clumps of moss peeling off the surface along with a layer of sand.  This type of chemical weathering is known as chelation and results in the effective removal of the residual iron cement still holding the rock together after the calcite cement has been dissolved by groundwater.  The photo below right shows how “dry weathering” of a boulder of the same material can result in well-defined liesegang bands.  Highly concentrated iron has cemented these bands within the massive sandstone, and without the influence of groundwater, they are preferentially resistant to weathering, leaving them in bold relief.

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On Friday, we looked at some of the last steep areas we haven’t vistited north of the Middle Fork east of Shirley.  Definitely still have Witts Springs right down to the river there, but there is also a thin- to very thick-bedded unit above it that is probably in the Bloyd.  We saw a fairly recent landslide above the river composed of material from that upper unit.  There was also a good cut and fill channel bed exposed in that unit as well.

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It was warm enough for the critters to be out again this week.  Just when I thought it was safe to put my foot down anywhere I pleased, I nearly stepped on a moccasin.  That’s him slinking back in his hole.  We also saw a western slimy salamander (plethodon albagula?) under some storm debris, which was subsequently replaced.

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Hopefully the warm weather holds out, but the forecast says the bottom may drop out on Friday.  We’ll see!

Until next week, see you on the outcrop!