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Notes on Quaternary landforms, Pendennis headland, Falmouth.

Last Updated: 7th Oct 2010

By Robert Broughton

Activity title Geology/Fossil trip (self organised)
Start Date 06/Oct/2010
End date 06/Oct/2010
Activity description: Personal field trip investigating Ipswichian deposits below Pendennis Castle approx~ 120,000 BP.
Compare with similar deposits studied recently at Saunton Beach, N. Devon.

Conclusions and initial assessment:

The only geological feature seen that may be attributed to the ipswichian interglacial is a relict wave cut platform east of coastgaurd station (SW 826 319). This feature is overlain by head deposits and is ~25' above mean water mark.

The buff cliffs that initiated this trip are composed entirely of unconsolidated head deposits of varying thickness. The very bottom layers (~40 - 60 cm) are stratified and exhibit upwards fining bedding. In places (e.g. SW 822 317, adjacent concrete revetment) a distinctive bed of white quartz pebbles can be seen. Observable thickness Β»10cm. These relict pebble "beaches" appear to be analogous with current deposits to the east of the coastgaurd station e.g. those at Crab Quay. They are roughly at current maximum high water mark and show that sea level occupied the current shoreline in the past, i.e. our current shoreline has been shaped by atleast two interglacials. This is supported by evidence from the Saunton sands trip where fossil barnacles were found attached to the shoreline rocks within the base of later transgression sandstones (~120,000 yrs, although this date is uncertain in the literature). At Saunton Sands it is thought this shoreline represents sea-level during the Hoxnian (425-375,000 b.p.).

No fossils were found in these lower stratified layers, although, if the beach hypothesis is correct, then small abraded shell fragments may yet be recovered. There certainly does not appear to be the shoreline fauna present that was recovered from Saunton Sands.

The upper thickness of the head deposits consists of soil and rock fragments containing 'heave' structures associated with periglacial action.

Dating:
The falmouth Ria system formed between 6-4,000 years ago. All structures seen must predate this.
Relict WCP must predate the head deposits.
Stratified head deposits and relict pebble beaches must predate periglacial action. They are several feet lower than the WCP and so are not likely to be contemporary, therefore they must be attributed to a subsequent transgression. However the current WCP shoreline must have been in existence at the time the lowest head deposits were laid down, and therefore may predate the raised WCP.

The current shoreline may date back to the Hoxnian interglacial if it is contemporaneous with that at Saunton Sands.

WCP is thought to be potentially ipswichian as its height above sea-level corresponds well with sites of known date at Saunton Sands and Westward Ho! Hence this puts it in the range of 120 - 70,000 bp.

The devensian glaciation between 70,000 and ~10,000 is a likely candidate for the unconsolidated head deposits, with the relict shoreline features marking the start of a marine regression. Periglacial action ended around 10,000 bp. The early Holocene (younger Dryas in US) would again have seen the start of the marine transgression that resulted in the formation of the falmouth Ria system.

Next tasks:
Revisit sites with camera and record various beds.
Attempt a more accurate elevation for the relict WCP for comparison with those in North Devon.
Read more literature regarding Quaternary sea-level fluctuation.





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