Sunday, 16 October 2016

Geophysics at the Gorgan Wall, Iran

Hunting the Red Snake – part II

Back in Iran, this time with my brother, Martin, to continue the hunt for the remainder of the Gorgan Wall.  Last year I was here with marine geophysical equipment to try and find the remains of both the Gorgan Wall and the Tammisheh Wall as they enter the Caspian Sea as part of a project with the University of Edinburgh and the National Museum, Iran.  From a previous blog (GorganWall I) I showed the results for the Tammisheh Wall extending well over 2km from the southern shores of the Caspian into a large lagoon behind the Miyankale Peninsula.  The sidescan sonar survey and sub-bottom profiling showed the remains of the brick wall with its accompanying ditch/canal.  Unfortunately we so no manifestation of the Gorgan Wall extension – so back to try again.

The Gorgan wall projection to the west towards the Caspian Sea
The Gorgan Wall (the Red Snake) extends from the Alborz Mountains in the east across the Gorgan Plains and was built from the 4th C AD as a defensive structure to keep out the marauding Turks from the north.  The wall was constructed of bricks fired from local clay in kilns spaced 40-80m apart along the length of the wall. It was at least 2m wide and tall expanding to a more substantial feature at numerous towers and where defensive forts occurred.  In front, to the north, of the wall was a 10-30m wide, up to 3m deep, canal.  The remains of the wall and canal are relatively well documented to the east. H,owever to the west, evidence of it remains elusive with historical writings describing the wall splitting into different parts and extending down to long-lost towns.

Despite being of similar age to the Tammisheh wall and being relatively close to each other the geomorphological setting for both is somewhat different.  The Tammisheh wall extends into the Caspian at a place where the sea is relatively sheltered with fairly benign sedimentation gently silting up the almost closed lagoon.  The projection of the Gorgan Wall however is across an area of coast where there have been highly dynamic changes due to the interplay between the Gorgan Ricer discharge and longshore drift bringing sediment onshore from the Caspian.  The present day shoreline demonstrates these active processes where the waves can reach impressive magnitudes due to a fetch stretching the length of the Caspian.

CMD Mini-explorer and Explorer ground conductivity meters
This year we have returned to attack the challenge of finding the remaining western part of the Gorgan wall as it progresses towards the Caspian Sea. We come armed with electromagnetic instruments, the GFInstruments CMD Explorer and Min-explorer.  Both are frequency domain ground conductivity meters, each with three coil spacings, in order to look to a range of depths.  We have a Leica dGPS for positioning control and for ground truth work a more traditional shovel and trowel.

The western limit of the wall is manifest by a series of “robber” pits that were used by locals to mine the decaying bricks for use in other projects.  The wall and remains of the canal that ran along the north side of the wall run in a straight course here for at least 20km.  Our first geophysical grids were laid over the robber pits and known wall/ditch locations in order to characterise their geophysical signature. The figure below shows this mapped on the satellite images.  Note that the background satellite images clearly define the agricultural system of 200x200m fields and also parts of the old natural landscape with its river channels and back barrier bars with sloughs.
EM Conductivity over robbed-out portion of wall
We continued to map the wall to the west and after approximately 1km a large modern drainage ditch was encountered and gave us the opportunity to ground truth the geophysics.  A 2D interpretation of the geophysics together with the surface mapping suggested that wall and ditch might exist here.  Excavation proved this to be true and samples have now been taken for analysis.

Using this characteristic signature mapping was continued a further 5km to the west with a complex pattern developing of linear features mixed with sinuous features.  Our current thinking has an interpretation based on a landscape evolution of shorelines behind which the old Gorgan River fights its way to the sea and through which the wall and canal/ditch is cut.  It is likely that the dynamic natural landscape was always an issue for maintaining the wall and canal here and a much larger geophysical and ground truth investigation is going to be necessary before a clearer picture of the history is revealed.
New geophysical results showing extension to the west from last known position

So, more geophysics to come hopefully in 2017. 

During the trip we also tried making maps with and AUV operated by Georgian colleagues.  While the purpose was to make maps along the wall projection we also took time to investigate some of the more interesting geological features.
Dormant mud volcano,Gorgan, Iran

This was way too short a trip as usual but a highly enjoyable one.  Our biggest issue this time was getting the equipment through customs – lessons for the next trip.  Our biggest hazard encountered (apart from the driving)  ........... watch out for those camels!


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