| Computer Code: |
PZS |
Preferred Map Code: |
PzS |
| Status Code: |
Full |
| Age range: |
Frasnian Age (DR)
— Famennian Age (DA) |
| Lithological Description: |
Cleaved purple mudstones in which bedding is picked out by pale green and grey beds and laminae, and by thin sandstone (some graded bedding, load structures and rare cross-lamination) and laminated siltstone horizons. Thicker beds of green mudstone occur in association with dark grey mudstone which is sporadically fossiliferous. Thin chlorite-rich tuffaceous mudstone beds seen in coastal exposures. Local occurrence of small dolomitised limestone nodules in beds up to 3 m thick. |
| Definition of Lower Boundary: |
Transitional with the underlying Harbour Cove Slate Formation. The boundary is placed where the colour changes from dominatly grey-green (Harbour Slate Formation) to purple (Polzeath Formation). In coastal sections, the base of the Formation is faulted. |
| Definition of Upper Boundary: |
Conformable with the overlying Gravel Caverns Conglomerates at Pentire Haven [SW 9333 7973]. In the field this tranistion is observed as a gradational change from cleaved purple mudstone with pale green and grey beds and laminae to conglomerate beds (up to 2 m) with interbedded grey-green mudstone. |
| Thickness: |
Up to 320 m. |
| Geographical Limits: |
North Cornwall, in the area around Pentire Point. |
| Parent Unit: |
Not Applicable (-)
|
| Previous Name(s): |
Upper Devonian Purple and Green Slates [Obsolete Name and Code: Use PZS]
(-475)
|
| Alternative Name(s): |
Polzeath Slates
|
| Stratotypes: |
| Type Section |
Between the north side of Daymer Bay and Greenaway Beach. |
| Reference(s): |
| Gauss, G A and House, M R, 1972. The Devonian successions in the Padstow area, north Cornwall. Journal of the Geological Society of London, Vol.128, 151-172. |
| Selwood, E B, Thomas, J M, Williams, B J, Clayton, R, Durning, B, Smith, O and Warr, L N, 1998. Geology of the country around Trevose Head and Camelford. Memoir of the British Geological Survey, sheets 335 and 336 (England and Wales). |
| Reid, C, Barrow, G and Dewey, H. 1910. The geology of the country around Padstow and Camelford. Explanation of sheets 335 and 336. Memoir of the British Geological Survey, England and Wales. |
| Beese, A P. 1984. Stratigraphy of the Upper Devonian argillite succession in north Cornwall. Geological Magazine, Vol. 121, 61-69. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
|
E336
|
