Geological History
The geological history of the Plym Valley is complex, although in its route from it's Dartmoor source to the sea the Plym only crosses three basic rock types: granite, slate and limestone and the cycle route lies almost entirely on slate.

The purple/grey slateswhich you will see in cuttings were deposited as mud in marine basins, which were then in tropical latitudes 360-375M years ago. The resulting rock sequence is known as the Upper (i.e. most recent) Devonian. The Devonian was named by the 19th century pioneer geologists Rev. Adam Sedgewick and Sir Robert Murchison after they recognised that most of Devon's rocks represented an intermediate stage between the earlier Silurian and later Carboniferous periods.

The subsequent cycles of deposition (perhaps thousands of metres thick), uplift and erosion over 360M. years has left virtually no trace of more recent strata, which have all been eroded by the actions of wind, rivers and ice and changes in sea levels.

Apart from the low quality slate (locally termed shillet), there are a number of places where much later intrusive rocks are visible (see maps). Dolerite, a hard grey/green fine-grained rock was worked at several small quarries for roadstone or railway ballast and a number of microgranite (elvan) dykes have been intruded E-W across the valley.

It is only at the mouth of the Plym that the Plymouth limestone outcrops. This resistant pale grey rock is the product of marine sponges and other organisms, similar in some ways to the formation of a modern coral reef.

Mining and Quarrying
Granite
The Dartmoor granite, known for centuries by locals as 'moorstone' is a valued building stone. The Foggintor, King's Tor, Royal Oak and Swell Tor quarries to the West of Princetown supplied the stone used in many public buildings- even Nelson's Column used granite from Foggintor.
Transport from the high moors down to Plymouth was always difficult, initially by horse, then by the Plymouth and Dartmoor Railway and finally by the one of several railway enterprises using the Plym Valley.

Slate
Slate of sufficient grade to be used for roofing Plympton Shcool in 1664 was worked at Cann Quarry, although it appears that most of the slate was used for flooring and walls. You can access the quarry from Cann Viaduct. Remains of a large water wheel pit(photo) and crushing machinery have been preserved on the quarry floor with information boards.
If on a bike lock it to the railings on the viaduct or carry it down the steep path to the quarry floor.
Dolerite
Small dolerite outcrops have been worked for roadstone and possibly railway ballast at several locations. Both car parks on the W. side of Plym Bridge are old dolerite quarries.

Shaugh Mining Area
Shaugh Bridge, where the Meavy and Plym meet involves a 300m detour from the cycle path at Shaugh Holt platform.

As well as waterfalls over huge granite boulders,the Dewerstone woods and high rock outcrops, this area includes a wealth of industrial archaeological evidence.
At various times in the last 200 years a ferro-ceramic mine, brickworks, quarries and a china clay drying plant have operated here.

read a fuller account by Steve Roberts of Plymouth Mineral & Mining Club

Transport routes
The cycle path to Clearbook mostly follows the Plymouth to Launceston Branch of the Great Western Railway, (opened 1883 - closed to passengers 1962) although to the S. of Plym Bridge the situation is more complicated.

Here the Plym Valley Railway occupies a section of line as their HQ and retain permission to open up the trackbed as far as Plym Bridge, so the cycle path from Marsh Mills to Plym Bridge is laid over a section of the former Lee Moor Tramway.
An earlier line, The Plymouth and Dartmoor Railway, had opened in 1823 to King Tor Quarry, near Princetown.
n.b. this line reached Yelverton via a convoluted forest and moorland route to the W. of the Plym. It closed in 1869, part of the trackbed subsequently being used for the tramway.

The Lee Moor Tramway opened in about 1858 to carry china clay and other minerals from Lee Moor on the S. side of Dartmoor to the quays in Plymouth initially by horse, then by a mix of horse, gravity and steam power. It included a 2km long 1:11 counterbalanced cable incline through Cann woods. You can see the bridge which carried the tramway over the road veering off to the NW at Plym Bridge.

From 1947 until closure in 1960 only the lower section of the tramway was in use. Some years later pipelines which carry the china clay in slurry to a drying plant at Marsh Mills were constructed.

The Lee Moor locos were rescued and restored between 1965 and 1975. Lee Moor No.2 is on display in Saltram House, across the Plym from Marsh Mills and Lee Moor No.1 is on display at the Wheal Martyn China Clay Museum, nr. St Austell.

Adjacent to the cycle path on the East side is a narrow canal. This was part of Lord Morley's Cann Quarry Canal and Cann Quarry branch line combined scheme, completed in 1829 to bring slate of paving slab quality down to Plymouth.
A lead/silver lode (mineralised vein) was revealed near Plym Bridge during excavations to build the canal. A small lead mine known as both Cann Mine and Canal Mine was developed and operated between 1824-5 Ruins relating to this venture can be seen close to the track.

The canal section was only used as such for less than 10 years, from when it was used purely as a leat and the railway line extended for the full distance, thus avoiding the need to transfer the cargo from rail wagons to the small tub barges used on the canal.

Viaducts, tunnels and platforms
Leighbeer tunnel (photo), also referred to as Leebeer or Shaugh tunnel is the only railway tunnel on this stretch of the path. Funded by Lord Morley and designed by Brunel, it was completed in the 1850's and is 281m long. Bat friendly lighting was installed in 2000 and the tunnel was resurfaced in concrete.
An aqueduct crosses the line near the S. tunnel portal. This carried a leat the Wheal Lopes copper mine, 1km to the SW.

4 viaducts carried the path over deep wooded valleys.
3 are between Plym Bridge and Bickleigh:
Cann viaduct (photo) is 116m long and 19m high. Originally built of timber, it was replaced by the present Staffordshire blue brick viaduct in 1907
Riverford viaduct (photo) is 116m long, 97m high and has 5 granite arches, replacing the earlier timber structure in 1893.
Bickleigh viaduct (photo) is 153m long, 38m high and has 7 granite arches, replacing the earlier timber structure in 1893.

Ham Green viaduct (photo), to the North of the on-road section is the longest at 174m and 28m high. Again, originally a timber construction, the present 6 arch structure dates from 1899.

Shaugh Halt platform (photo) was opened for passengers in 1907 on the site of a works siding used during the construction of the tunnel. This was also the point where dried china clay was loaded from the Shaugh Bridge plant.
Clearbrook Halt served Clearbrook, uphill to the W, as well as Hoo Meavy and Goodameavy. Opened in 1928, it remained in use until its closure on 31 Dec. 1962.

Plympton
Plympton was a thriving market town centuries before Plymouth was anything more than a collection of villages. In 1194 Plympton was made a stannary town, which meant that all tin from SW Devon had to be weighed there. Most of the tin would have been carried by packhorse over rough moorland tracks. The Tory Brook branch of the Plym Estuary was navigable right up to the centre of old Plympton and a record of 1178 lists a shipment of slate from Plympton to Southampton.
home