Striations

   

Glacial Prehistory 

Striations 

  

Striations are features present on some stone surfaces that have grroved surfaces that may have been eroded by glacial ice streams. They are present under materials that may be glacial, and if they have not been scarred by this material its difficult to see how they were deposited. 

This is a record of some striations,  just one at the moment, it is on South Ronaldsay in Orkney. 

  

The illustrated location is in a bay in South Ronaldsay on of the islands of Orkney. The orientation and context of this sample feature is not a common occurrence in this location, but is an outcrop of a hard fine-grained rock which responded to abrasion by allowing its surface to be eroded with detailed lineations. 

Elsewhere softer or flakier materials simply broke up as material flowed over it.

   

  

 Above is the location of the the Orkney Islands north of mainland Scotland with location of St Margaret's Hope where the feature is to be found.

The arrow indicates the orientation of the striations.  The angle of fall of the striated rock surface is about 5 or 10 degrees, dropping towards the sea.

 

Above is the outcrop from the beach. 

Closeup of the outcrop striation group.

   

Above is the striated stone.

     A view, in the direction to which the striations are pointed. It shows the port of St Margaret's Hope, with the Pentland Ferries,  roll-on roll-off ferry Pentalina, in port.

  

The sketched chart shows island of South Ronaldsay with the location and orientation of the striated stone.

Also shown are the ridges of a series of parallel valleys crossing the island.

From "Bedrock, Ice age and Holocene  Environment in Orkney" by John Brown.

Chart shows striations across Orkney generally following a southeasterly to north-westerly direction. 

The flow direction of the striations on the bedrock stones are in complete contradiction to the general topography over the archipelago. Specifically, the flow of ice over the South Ronaldsay ridges and valleys is not tenable. 

Jeffery Nicholls 

Orkney 

Jiffynorm@yahoo.co.uk 

#geology #Quaternary #Orkney #Scotland