Patterns and rates of erosion produced by high energy wave processes on hard rock headlands: The Grind of the Navir, Shetland, Scotland

A. M. Hall, J. Hansom, J. Jarvis

Research output: Contribution to journalArticlepeer-review

70 Citations (Scopus)

Abstract

The Grind of the Navir is an ignimbrite headland on the exposed Atlantic coast of the Shetland Islands, Scotland. During storms, offshore wave heights exceed 20 m and deep water close inshore allows high-energy waves to impact on these cliffs. The cliff top at similar to 15 m above sea level is awash with wave water when wave heights exceed 8 m, a condition met in storms in most years. Detailed mapping, ground photography and patterns of lichen colonisation and growth allow the processes, patterns and rates of erosion to be assessed on different parts of the headland over the past 100 years.

Recent fractures in the rock of the cliff face and top indicate that wave impact forces exceed the 1.5 kPa tensile strength of the rock and fresh sockets low on the cliff face record the removal of fracture-bounded blocks that are lost to seaward. On the upper part of the cliff face, upward moving water flow is capable of removing blocks from both vertical faces and stepped overhangs. Clefts in the cliff face are extended inwards and upwards by crack propagation and block removal, leading to the development of slot caves.

The movement of waves across the cliff top in fast-moving bores quarries blocks from the cliff face close to the edge and from rock steps on the cliff top, promoting further rock fracturing. Socket sizes and block characteristics indicate that blocks of > 1 m(3) are rotated from sockets on the cliff top platform and carried landwards for up to 60 m to be deposited in a series of boulder ridges on the cliff top. Multiple boulder movements during a storm in January 2005 generated impact marks and orientated striations that allow the pattern of water movement to be reconstructed across considerable areas of cliff top.

The average rate of erosion at The Grind is estimated at 1.3-6.0 mm/yr on the cliff face and 5 mm/yr on the cliff top, although erosion rates vary in both time and space. Although block movement on the cliff top occurs in most years, erosion of rock by block removal is most active during major storms. Nevertheless, some parts of the cliff top and face remain blackened by lichen and so have experienced little recent erosion over the last 70 years or so. Although erosion occurs at the base of the cliffs, in recent decades it has been concentrated on upper part of the cliff face and top. Cliff-top erosion is most intense where stepped geos act as channels for waves to access the cliff-top. The most active features on the cliff top are the boulder ridges where substantial changes can be traced to storms over the past century. Unlike conventional models of cliff erosion where erosion is concentrated by wave attack at the waterline, wave impacts on this headland may occur on any part of cliff face and top and appear particularly effective on the upper cliff and cliff top platform. (c) 2007 Elsevier B.V. All rights reserved.

Original languageEnglish
Pages (from-to)28-46
Number of pages19
JournalMarine Geology
Volume248
Issue number1-2
DOIs
Publication statusPublished - 8 Feb 2008

Keywords

  • cliff-top storm deposits
  • wave dynamics
  • marine erosion
  • coastal evolution
  • rock coast
  • block quarrying
  • Scotland
  • Shetland
  • DEPOSITS
  • IRELAND
  • COASTS

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