A Drowned Landscape EarthCache

The precise date is controversial but certainly in Neolithic times, and possibly as recently as the medieval period, you would now be standing in the middle of one island that now forms Tresco, Bryher, Samson and the Tresco channel. Rising sea levels since the most recent ice age maximum have left a drowned ancient landscape. (Thomas (1985))

Writing in 1753, William Borlase comments that, “the flats … are plain evidences of a former union subsisting between many now distinct islands. The flats between Trescaw, Brehar and Sampson are quite dry at a spring tide, and men easily pass dry-shod … where, upon the shifting of the sands, walls, and ruins are frequently discover’d.” (Borlase 1753: 57)

Well done and thanks to heartradio for making the FTF on what might have been the first feasible day.

Tide times and expected heights can be found in various places:

• Posted outside the office on St Mary’s quay.
• In annual booklets sold in, amongst other places, Mumford’s shop in Hugh Town.
• On various websites such as (I cannot vouch for the reliability of these sites):
  • http://www.visitmyharbour.com/tides/1/uk-tables/stmarys-scilly-isles-tide-tables
  • https://www.tidetimes.org.uk/st-marys-tide-times

SEA LEVEL CHANGE MECHANISMS

• Eustatic Variation is due to changes in the volume of the Earth’s seawater. (Peltier (1998:603)) Often this is due to warming and melting of ice caps after ice ages, in which case sea levels rise.
• Isostatic Variation is due to changes in the Earth’s shape- played out through movement of the crust. One mechanism for this kind of change is “isostatic glacial rebound”. (Peltier (1998: 607)) This occurs when melting ice sheets reduce the weight of material lying on the crust, causing it to rise and hence sea level to fall. Previously, sea levels would tend to rise as the weight of ice built up.

In Scotland, in recent geological time, these two processes have acted in opposition, with post-glacial isostatic uplift of land but eustatic increase. It is the isostatic process that has been more pronounced and hence sea levels have fallen since the last ice age, leaving raised beaches. (See Earthcache GC6B18N )

By contrast, in the Isles of Scilly the two processes have acted in the same direction. There is more water in the oceans since the last glacial maximum approximately 19000 years ago (Clark et al (2009)) and the crust has also dropped. (The glaciers in the last ice age did not reach the Scillies and as the North of the British Isles rebounds up, the crust “forebulge” in the SW of England drops. (Peltier (1998: 610)))

SEA LEVEL RISE IN THE ISLES OF SCILLY

Sea level has been rising since the most recent glacial maximum approximately 19000 years ago. (Clark et al (2009)) 5000 years ago, early Bronze-age inhabitants of Scilly would have been farming across a low-lying plain between what are now St Mary’s, St Martin’s, Tresco and Bryher. (Robinson (2013)) Only St Agnes, Annet and the outlying rocks would have been separate islands at that time. Across the islands there is evidence on the current shoreline of past human activity that has been inundated. For instance, Bronze-age hut circles can be found in the low cliffs on the North side of Porthcressa bay and at Porth Killier on St Agnes. (Ratcliffe & Straker (1996)) Note that sausage sarnie’s Earthcache Eustatic Scilly is at the nearby raised beach at Porth Killier, which was formed millions of years ago when sea level was higher than today.

The date at which the large island became many is somewhat controversial and, due to the ocean’s tidal nature, necessarily inexact. In terms of the Tresco channel being flooded more than dry, Thomas claims that it happened as recently as the 16th century. Before then the Tresco-Bryher island was “St Nicolas Island” and, before that, Tresco-Bryher-Samson was, in 800 AD, “Rentemen”. (Thomas (1985: 246)) "Breyher" was the name for the hilly Eastern side of this larger island. Thomas’s model of sea level rise in Scilly claims a 7.25m rise since 1000 BC. (1985: 27) His model is based on the assumption that the lowest altitude habitation remains at roughly the same height above sea level over time and is inferred from dated features such as the hut circles mentioned above. Thomas supports his claim for a late date for inundation of the main island on the basis of the names of geographical features. Pre 1600 names are Cornish and after that time English is the dominant language. Marine features such as rocks and beaches tend to have Cornish names where they lie on the outer coasts of the current islands, whereas the coastal features facing inwards, towards what was the low-lying plain, have English names. For instance, “Porth-“ as a prefix is Cornish and “–porth” as a suffix is English. All “Porth- s” are found facing away from the intertidal area (and on St Agnes), while “-porths” and “bays” are found pointing towards the centre. (Thomas (1985: 39))

More recent studies suggest that Thomas’s model overestimates the rate of sea level rise. Ratcliffe & Straker provide evidence of a 2.55m rise since 1000BC. (1996: 50) This implies a much earlier date for the flooding of the Tresco channel. Their evidence is based on “intertidal” peat deposits below current high tide levels. They study deposits on beaches such as Porth Mellon on St Mary’s and Par Beach next to the Higher Town quay on St Martin’s. The peat is analysed to determine whether it was formed in salt water, brine or fresh water and then samples are carbon-dated. (1996: Section 4) Robinson (2013) includes an interesting animation of 11000 years of sea-level change based on even more recent evidence. http://intarch.ac.uk/journal/issue34/2/images/animation1.html

REFERENCES

SAFETY

Many hundreds of people walk across the channel every year and Tresco’s owners suggest it as an activity for visitors. http://www.tresco.co.uk/island-news/walking-to-bryher/ We have made this walk dozens of times, including with toddlers. Nevertheless, there are potential dangers and you attempt this Earthcache at your own risk. The difficulty rating reflects the fact that the cache cannot always be found. The terrain rating reflects the tidal nature of the cache but at a low spring tide the terrain is really more like 2.5 or even 2. Our non-professional advice is:

• Make sure that you understand how tides work and that you know the time and height of the low tide (confirming that it is a spring tide). The cache is not reachable at neap or normal low tides. You can only get there at a spring low tide although it need not be an extreme spring tide. Tides of 0.7m or lower are likely to be straightforward but tidal prediction is not an exact science and the level of the water is affected on the day by air pressure, wind or storm surges so that figure is not a replacement for making your own judgement.
• Make sure you know whether the tide time that you have looked up is GMT or BST and which is in operation at your time of year. Make any necessary correction.
• Take a mobile phone and be ready to call 999 in an emergency and request the Coastguard.
• Do the cache in a group.
• Mist would make the cache more dangerous.
• Spring tide low tides are always around midday and midnight (GMT). Doing the cache at night time is not advised.
• Use your common sense at all times.

LOGGING QUESTIONS

1. At the posted coordinates please take an altitude reading for the sea bed. (The sand, rather than for your GPS at waist level.)
2. On Bryher, at N 49° 57.023 W 006° 21.121 look towards St Mary’s. Please describe in detail, including rough measurements, what you can see on the beach immediately in front of you and say what you think you are looking at. Hint: the feature is connected to the Earthcache's theme.

OPTIONAL NON-LOGGING REQUIREMENTS

It would be great if finders could put the following information in their logs in order to build up a record of the relative water levels at different low tide heights. It will also give future finders an idea of what conditions to expect.

• A photo of the channel and the time of your visit.
• The time of low tide and its height above datum on that day.

Please provide answers by message to the cache owner. Feel free to log the cache immediately but false, overly vague or no answers received will result in a deleted log. YOU MUST VISIT ON FOOT AT LOW TIDE. Please avoid spoilers in your log.