Cholwell Brook

Site of Special Scientific Interest · 50.6102, -4.1072

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Also within

Devon County DARTMOOR National Park

Condition (Natural England)

Favourable (1)
UnitFeatureConditionArea (ha)Assessed
SX510811 — Favourable 0.46 2011-12-16

Habitats

Fast flowing river and stream

Geology & landscape

Geological Feature: Igneous Intrusion · Geological Period: Carboniferous · Rock Type: Chert, Limestone

Landscape processes: Volcanic / Igneous

Description

Cholwell Brook SSSI Description A Geological Conservation Review Site. This reach of the Cholwell Brook lies within Dartmoor National Park, approximately two kilometres north-east of Mary Tavy on the Tavistock–Okehampton road (A386). The site encompasses a narrow section of stream within a south-west–north-east orientated valley between Gibbet Hill and Kingsett Down. The brook is a tributary of the River Tavy, which it joins approximately two kilometres further south between Peter Tavy and Mary Tavy. This site includes a stream section through deformed, interbedded, turbidite sandstones and shales of the Upper Culm Crackington Formation (Namurian) and shows the thrust contact of this formation with the Lower Culm Brendon Formation beneath. The Crackington Formation is inverted and demonstrates the fold nappe concept applied to this sequence by recent workers. Small scale sedimentological features, cleavage-bedding relationships and fold shapes are well exposed, demonstrating way-up, younging directing, structural facing and vergence. The lower thrusted boundary to the formation is marked by about 50 metres of tectonic melange consisting of clasts of sandstones, chert, dolerite, limestone and volcanic rocks up to tens of metres in length, set in a soft clay gouge matrix. The fault zone, and the transition into it, is clearly exposed in the stream section. This thrust is one of the most important tectonostratigraphic boundaries of south-west England, separating the structurally higher Carboniferous sequences from older, more deformed, sequences beneath.

Plant communities

A9 — Fast flowing river and stream, Marginal and inundation, Pond ditch and canal, Sluggish river and stream

Potamogeton natans community

Schoenoplectus lacustris, Juncus bulbosus, Sparganium angustifolium, Sparganium erectum, Sparganium minimum, Alisma plantago-aquatica, Lobelia dortmanna, Equisetum fluviatile, Myriophyllum alterniflorum, Elodea canadensis, Lemna minor, Lemna trisulca, Nymphaea alba, Callitriche platycarpa, Callitriche stagnalis, Littorella uniflora, Potamogeton natans, Ranunculus trichophyllus, Glyceria fluitans, Chara subspinosa

A14 — Fast flowing river and stream

Myriophyllum alterniflorum community

Isolepis fluitans, Juncus bulbosus, Fontinalis antipyretica, Lobelia dortmanna, Equisetum fluviatile, Myriophyllum alterniflorum, Lemna minor, Callitriche stagnalis, Littorella uniflora, Potamogeton filiformis, Potamogeton gramineus, Potamogeton natans, Ranunculus peltatus, Chara spp., Nitella hyalina

A17 — Chalk river and stream, Fast flowing river and stream

Ranunculus penicillatus ssp. pseudofluitans community

Berula erecta, Nasturtium officinale, Elodea canadensis, Lemna minor, Callitriche stagnalis, Veronica beccabunga, Polygonum amphibium, Polygonum hydropiper, Potamogeton crispus, Potamogeton pectinatus, Ranunculus penicillatus ssp. pseudofluitans

A18 — Fast flowing river and stream

Ranunculus fluitans community

Fontinalis antipyretica, Fontinalis squamosa var. curnowii, Myriophyllum alterniflorum, Myriophyllum spicatum, Elodea canadensis, Mentha aquatica, Lemna gibba, Lemna minor, Potamogeton perfoliatus, Ranunculus fluitans, Glyceria fluitans, Phalaris arundinacea