Lustleigh
Cleave from Harton Chest
If you have arrived at the co-ordinates as listed, you are now
standing on a granite tor called Harton
Chest (Raven’s Tor), and looking out onto a valley called
Lustleigh Cleave. This Earthcache will reveal a little about the formation
of this steep sided valley.
Please note that Harton Chest is a tor at some height above ground
level with a steep drop at one side. Although not a difficult
climb, if you are in any doubt about your ability to make your way
to the top of this tor, please do not attempt it. The questions can
be answered and the Earthcache claimed
without having to climb the tor, although of course you will miss
out on the spectacular view. If you decide to climb, then you
assume all responsibility for this.
There is on-road parking at N.50.37.218 W003.43.962. Follow the
public footpaths through the woods and uphill to the ridge of the
Cleave.
History
During the Devonian period (c. 395 . 345
million years ago) and the Carboniferous Period (c. 345 – 280
million years ago) rocks such as sandstones, shales and limestones
were formed by deposits from the tropical seas that covered this
area.
Then around 280 million years ago the sea floor experienced a
period where these deposits were squeezed together, and slowly
folded into mountain ranges. A large area of magma (molten rock)
beneath Dartmoor was intruded into the rocks below these mountains,
and cooled to form granite. This mountain building period caused
the plain of Dartmoor to tip downwards from north to
south.
The Sticklepath Fault is thought to have formed in the early
Tertiary Period (c. 65 – 50 million years ago), and is a
wrench or tear fault. A fault occurs where tectonic forces acting
beneath the earth’s crust cause a fracture to occur at the
surface. It is possible that the Sticklepath Fault originated from
the pressures of the formation of the Atlantic Ocean through
sea-flooring spreading. A wrench fault occurs where the two sides
of the fracture move laterally in opposite directions as shown in
the diagram below. The Sticklepath fault runs NW to SE in
direction, and is thought to start at near Bideford in the North,
running across the NE corner of Dartmoor and through to Torquay in
the South. It is thought to be responsible for the earth tremors
that were felt throughout the region in November 1955.
Wrench or Tear Fault
Cleave Formation
This lateral movement of the earth’s crust displaced the NE
corner of the granite intrusion as shown in the diagram below. It
is thought that the displacement is approximately 1 mile in
length.
Diagram showing the geology of North-East Dartmoor and the
Sticklepath Fault Line
Lustleigh
Cleave is the area marked A on the diagram. On the NE side of the
Cleave the rock beneath is granite. However, on the other side of
the valley the rock is hornfelsed
sediments called Culm. The Cleave itself is described as a
fault-line scarp. Scarps such as these are formed when erosion of
weaker rock (the Culm in this case) occurs at a faster rate than
that of the harder rock (granite). This has resulted in the steep
face scattered with granite boulders, dropping down to the lowered
floor of the valley beneath, on the other side of the
fault.
In order to log this earthcache, please answer the
following questions.
1. Standing on Harton Chest there is a tor in front
of you on the other side of the valley. Imagine you return to this
spot in 100 million years and further displacement along the fault
line has occurred. Would you expect this tor to have moved to your
left or right ?
2. The tor you are standing on is formed from
intruded magmas, forming granite. Look carefully at some of the
granite boulders. There are areas of feldspar crystals (called
phenocrysts). These are a white colour against the grey granite and
are rectangular in shape. Please can you give me some idea of the
range of sizes of such crystals that you see ?
Although not necessary to log the cache, we would
love to see a photo of yourself or your GPSr at the site. Many
thanks to Natural England for permission to place this
Earthcache.
References
Blyth, F.G.H. (1957), 'The Lustleigh Fault in
North-East Dartmoor', Geological Magazine 94 pp291-6
Natural England website -
http://www.english-nature.org.uk/Science/natural/NA_HAbDetails.asp?Name=The+Culm&N=93&H=65
Dartmoor National Park Authority Geology and
Landforms Factsheet March 2005
http://www.dartmoor-npa.gov.uk/__data/assets/pdf_file/0019/41266/lab-geology.pdf
Hesketh, R (2008), Devon's Geology - an
introduction, Bossiney Books, Launceston