HIDDEN OR BURIED KARST FEATURES WITHIN THE KARSTIC LIMESTONES
OF THE CITY OF PLYMOUTH.
24 April 2005.
1. INTRODUCTION :
The southern limit of the City of Plymouth is crossed from east to west by a belt of Middle Devonian Limestone, which for much of its length, is significantly karstified. This 6 mile (10 km.) -long belt of limestone runs from Empacombe, Mt. Edgecumbe and Cremyll in the west to Elburton and Sherford in the east, has an average width of about 0.9 mile (1.5 km.) and covers an area of about 5.8 sq miles (15 km²).
For the whole length of its northern side, it is bounded by and is contiguous with strata of Middle Devonian Slates and Shales. The structural contact between these two types of strata varies along the length of their contiguous northern boundary.
For much of the length on its southern side, it is exposed at the coast and forms the historically-important northern shoreline of Plymouth Sound. In the north-eastern corner of the Sound, the River Plym emerges having cut through the limestone mass in what is now a submerged limestone gorge deep below the Cattewater. In the north-western corner of the Sound, the River Tamar emerges.
At Stonehouse and at Sutton, the combination of both marine and land erosion processes have resulted in the sea having broken through the limestone on the southern side of its outcrop to connect directly with the Slates and Shales on its northern side.
Owing to the post-Devensian Glacial rise in sea level, the bulk of the carbonate strata is temporarily submerged. The result is that currently, we are only presented with an average vertical range of about 110 feet (33.5 m.) of exposed limestone above Ordnance Datum.
The Plymouth Limestones vary in depth across the width and length of the outcrop and are composed of various facies of differing solubilities. Primary porosity of the carbonate strata is generally low but as is usual in limestones, the secondary porosity is generally high. A complex karst hydrogeology has developed and of which field evidence supports the movement of groundwater through both fractured rock and conduit aquifers. It is the latter type which is more specifically characteristic of karst.
2. HIDDEN OR BURIED KARST FEATURES WITHIN THE PLYMOUTH LIMESTONES :
Across the width and along the length of the Plymouth Limestones, there are known to be localised "hotspots" where the degree of karstification has been enhanced, resulting in the develpment of caves or cave systems and associated surface karst geomorphological features. All too often in the past and as recently as 1999, various locations in our City's karst have been destroyed by quarrying, destroying the caves that they contained. However, in many other cases, the caves and their surface manifestation have only been obscured or buried below buildings or other developments. The process of obscuring the karst has been a gradual one and certainly has not been wholly limited to the 19th or 20th centuries.
This Section of the Society's Webpages is devoted to illustrating some of these interesting features by drawing on information kept within the Society's Archives.
We start the series with a modest and intriguing feature at the west end of the Plymouth Hoe Limestone Plateau, a veritable "hotspot" for underground karst development. In due course, we will offer other features in this area which, when seen together, will illustrate the existence and extent of the "West Hoe Cave System".
Please click on the Link below to access further information :-1. The Prospect Place Ponor, West Hoe, Plymouth, Devon, UK.
more to follow
|....Return to Education Homepage....|