Coniston Fossil Hunting

1873 – Ashgill Beds enters geological terminology
Salter used the name Ashgill Beds for the local upper Ordovician succession, an early step in the long history of the locality’s stratigraphical names.

1905 – The Ashgill Series is introduced
Marr introduced the Ashgill Series, taking its name from Ash Gill. The name’s origin here does not make this quarry the global Ordovician–Silurian boundary stratotype.

1913–1916 – The series type area and local section are clarified
Marr selected Cautley as the Ashgill Series type area and later published a detailed map and section of Ashgill Beck and the quarry in 1916. The type area and the locality that supplied the name are therefore distinct.

1948 – The Calymene Beds are redated
King and Williams established an Ashgill age for the older Calymene Beds near Ashgill Beck, correcting their previous Caradoc assignment. These beds are now placed in the High Pike Haw Member.

1954 – The geological site receives protection
Ashgill Quarry was first notified as a Site of Special Scientific Interest. Its protected boundary was subsequently revised in 1965.

1969–1970 – The Silurian graptolite succession is refined
Rickards developed the Wenlock biozonal framework and correlations for the Torver–Ashgill streams, revealing a finely divided fossil record distinct from the quarry’s Ordovician shell-bearing beds.

1974–1975 – Graptolites constrain the boundary successions
Hutt’s work helped establish latest Ordovician persculptus beds in the basal Skelgill succession at the quarry and documented the thin, less precisely dated late-Telychian band in Torver Beck.

1979 – A Torver trilobite subspecies is described
McNamara described the form originally named Tretaspis convergens deliquus, based on a holotype from between Torver Beck and its first tributary. Later revision changed its taxonomic placement.

1979 – Torver material contributes to a new trilobite species
The description of Primaspis bucculenta included Torver-region material. Its holotype came from Willy Scrow, so Ashgill Quarry is not the species’ type locality.

1980 – The Torver Tretaspis receives a revised placement
Owen reassigned the relevant Tretaspis convergens forms under Tretaspis hadelandica. The Torver subspecies is used in the later geological account as Tretaspis hadelandica deliqua.

28 August 1987 – The protected geological area is extended
The SSSI was renotified with a major extension and a minor deletion, protecting a broader landscape of quarry, stream and structural exposures.

1990 – Local type and reference sections are revised
Kneller’s lithostratigraphical work formalised the roles of local Brathay, Birk Riggs and Coldwell sections, improving comparison of the Silurian rocks across the Lake District.

1994 – The modern Dent framework is established
Revised formations and members separated the Ordovician Dent succession from younger Coniston terminology and placed the Troutbeck Member within the Ash Gill Mudstone Formation.

1999–2000 – The quarry and stream records are brought together
Detailed conservation accounts and the geological memoir synthesised the Ordovician quarry succession, the younger stream sections and their fossil and structural evidence.

2012 – The formal names are updated
The later framework refined the lithological names and recommended discontinuing Randy Pike and High Cross as formal members. Their real lower and upper calcareous intervals remain important features of the Coldwell succession.

The Coniston guide covers several separate exposures: Ashgill Quarry and Ashgill Beck, the younger succession in Torver and Ashgill becks, and a roadside ash-bearing exposure near the Walna Scar approach. Together they record part of the Ordovician–Silurian Windermere Supergroup. The measurements below belong to their named exposures; they do not form one continuously visible quarry wall.

BORROWDALE VOLCANIC GROUP

Older volcanic foundation. Around High Pike Haw, the older volcanic rocks were eroded before the younger Dent Group sediments accumulated. This foundation helps explain the upland landscape, but it is not a uniformly fossil-bearing part of the quarry succession. No single quarry thickness is established for it.

DENT GROUP

Kirkley Bank Limestone Formation

High Pike Haw Member

The historical Calymene Beds. Poor exposures in the Ashgill Beck area represent an estimated 40 m of calcareous siltstone, fine sandstone and nodular limestone. Their Cautleyan age spans shelly Zone 2 into the lowest part of Zone 3. Storm deposits are recognised in the member, and its trilobites have been grouped in a Proetid Association. The estimate describes the local succession, rather than 40 m of accessible rock in Ashgill Quarry.

Torver Member

The historical Phillipsinella Beds, 3–4 m. Cleaved calcareous siltstone occurs in the bank of Ashgill Beck about 15 m above the waterfall. These Cautleyan Zone 3 beds contain a mixed marine bottom-dwelling fauna, including a recognised Illaenid Association, and are older than the main quarry mudstones. The characteristic trilobite records include Phillipsinella parabola aquilonia and Tretaspis hadelandica deliqua; those names should not be applied indiscriminately to fossils from the younger Ash Gill Mudstone Formation.

Broughton Moor Limestone Formation

Limestone above the waterfall, about 4–5 m. Nodular to micritic limestone in Ashgill Beck is less than 5 m thick and is correlated with Rawtheyan shelly Zone 6. Local trilobite evidence includes sparse Diacanthaspis. The much richer faunas described from other outcrops of this formation are not a species list for this exposure.

Appletreeworth Volcanic Formation

Resedimented ash, 5 m. Just above the Ashgill waterfall, sand-sized volcanic material forms a roughly 5 m interval. It represents ash reworked and deposited by sediment gravity flows and is presumed to belong to Rawtheyan Zone 6. It is a separate volcanic episode from the Yarlside rocks of Longsleddale.

Ash Gill Mudstone Formation

Base of the Ash Gill succession. The regional base is an erosional boundary, but the amount of missing time at each local exposure is not quantified. A mapped or inferred boundary is not necessarily an obvious surface visible to a visitor. Ashgill Quarry is the formation’s type section.

Troutbeck Siltstone Member

Basal shelly beds, 4 m. Blue-grey calcareous mudstone and siltstone with shell debris form the waterfall and are also seen at the north-west base of the quarry. They belong to the latest Rawtheyan, shelly Zone 7. Seven trilobite species have been reported from the beck succession, although a complete, securely localised species list is not available here. Mucronaspis mucronata is among the trilobites recorded from the Troutbeck Member. Reports from overlying shales elsewhere do not establish a find in the younger quarry mudstones, and the old “Mucronatus Beds” name does not identify every fragment.

Ash Gill Mudstone Formation

Main Ash Gill mudstones, almost 20 m in the quarry. Cleaved mudrocks above the basal member contain a sparse Hirnantian Hirnantia brachiopod assemblage. The formation as a whole is described as about 25 m thick here, including its basal member; that total must not be added above the nearly 20 m quarry interval. Specific brachiopod lists from Appletree Worth Beck or Hol Beck describe other exposures.

A 1 cm bentonite in loose blocks. Altered volcanic ash has been found as a centimetre-thick layer in loose Ash Gill material at the quarry. Its original position has not been located in the face, so it cannot be assigned a fixed height within the quarry section.

Ash Gill–Skelgill contact. The south-east quarry face preserves the transition into the overlying Skelgill Mudstone Formation, described as conformable. Cleavage is conspicuous and can obscure bedding. The formation boundary is not itself proof of the Ordovician–Silurian system boundary.

STOCKDALE GROUP

Skelgill Mudstone Formation

Basal persculptus beds at Ashgill Quarry. Graptolites in the basal Skelgill succession show that these beds are still latest Ordovician, in the Hirnantian persculptus Biozone. No individual local bed thickness is securely established in the inspected account. A Stockdale Group label therefore does not make every bed here Silurian.

Quarry scree and loose spoil. Displaced material can mix the basal member, main Ash Gill mudstones and younger beds. A fossil-bearing slab without an observed parent bed cannot be placed precisely in this succession. The presence of loose rock does not establish permission to collect it.

Younger Skelgill beds in the Coniston area. Black graptolitic and paler mudstones extend into the Llandovery. An approximately 40 m thickness is quoted for the wider Coniston area, where detachment and deformation complicate the succession. It is not a measured 40 m wall at Ashgill Quarry.

Browgill Mudstone Formation

Pale mudstone and thin dark bands. Telychian beds in the streams are mainly pale green mudstone with thinner dark, graptolite-bearing intervals. Published district thicknesses of 50–90 m, and a roughly 7 m crispus interval, provide regional context rather than a complete measured Torver log. Stockdale Beck’s separately described 23 fossil bands cannot be transferred here.

Upper Browgill siderite concentration. The district account describes siderite concentrated about two-thirds of the way up the formation. This is a broader stratigraphical observation, not a separately measured bed at Torver. A locally identified graptolitic layer occurs above this level.

The 2.5 cm Torver graptolite band. At SD 2768 9607, a thin black layer was recorded about 5 m below the position then used for the Wenlock base. Its graptolites have been assigned, with uncertainty, to the griestoniensis and/or crenulata interval. The record does not prove that both biozones are present in the layer.

Red laminae near SD 2765 9612. A few red layers occur in the upper Browgill succession near Torver Beck. No local numerical thickness is established. They are not the thick red-bed packages described at some more easterly localities.

Far House Siltstone Member

Uppermost Browgill beds. The district succession ends with a pale interval assigned to this member. Its usual district thickness is 2–3 m, increasing to more than 10 m east of the Coniston Fault; neither is a new measurement of this particular stream section. No fossils were recorded from the member in the Ambleside district account.

TRANEARTH GROUP

Brathay Mudstone Formation

Dixon Ground Siltstone Member

Basal transition, 10–30 m in the Torver–Ashgill account. Relatively homogeneous grey siltstone passes into the darker laminated facies above. Burrow mottling is diffuse low in the member and better defined higher up, with laminated bands intercalated through the transition. The member lies in the centrifugus Biozone. Older descriptions called this Wenlock; the later British scheme places the Wenlock base at murchisoni, leaving centrifugus in the latest Llandovery.

Upper Dixon Ground marker: regional context. A slightly calcareous mudstone approximately 10 cm thick, with a characteristic pitted weathering surface, is described within the uppermost metre of the member in the formal framework. It has not been independently located in these streams and should not be confused with the younger nodule horizon above the member.

Brathay Mudstone Formation

Calcareous-nodule horizon above Dixon Ground. A separate 10 cm marker occurs about 15 m above the member’s top, at the top of the centrifugus Biozone. This is a distinct stratigraphical level from the marker within Dixon Ground.

Main laminated succession, 160 m locally. The Torver–Ashgill account records roughly 160 m of Brathay strata, largely dark laminated siltstone. Fine couplets of quartz-bearing silty sediment and carbonaceous mud occur at a scale of about three couplets per millimetre. That thickness is distributed through the stream and adjoining exposures, rather than visible in a single face. The later formal type section lies in Ashgill Beck at SD 2711 9525–2728 9505.

Graptolite preservation change. Around the middle of the murchisoni Biozone, preservation changes from pyritic relief below to flattened fossils above. It is a useful local correlation feature, but does not give a numerical thickness for the individual bed or identify a loose specimen by colour alone.

Thin sandstone influx and higher Brathay beds. Thin turbiditic sandstones appear in the riccartonensis Biozone. Upwards, the silt becomes coarser, thin mud-rich turbidites appear, and carbonate nodules grow larger and more frequent. Rare crinoids, corals and brachiopods become less scarce in this higher interval. Published bed counts and individual sandstone thicknesses are not available for every layer.

Higher graptolite intervals. The correlated succession includes the antennularius, rigidus, linnarssoni and lundgreni intervals. The intervening ellesae Biozone was considered likely but was not independently verified in the original study. Biozone labels are not a series of equally thick physical beds, and their complete species lists cannot be assigned to every exposure.

Thin ash bands beside the Walna Scar road. At SD 2889 9697, near the approach rather than in Ashgill Quarry, several pale grey ash-alteration bands weather orange. Most are less than 1 cm thick. They are separate from the centimetre-thick ash layer known only in loose Ash Gill blocks.

Lower part of the thicker roadside ash bed, 4 cm. The lower portion is pink-brown, loose and sandy, representing altered crystal-rich volcanic ash. It directly underlies the finer upper part described next.

Upper part of the same ash bed, 3 cm. Coherent pale yellow silty material contains thin lamination and cross-lamination, with fine grading near the top. Together the 4 cm and 3 cm parts form one 7 cm bed; they are not evidence for two separated ash horizons.

Birk Riggs Sandstone Formation

The local sandstone maximum, 360 m. The formation reaches its thickest development and type area at Birk Riggs, SD 2797 9537. It contains fine- to medium-grained, quartz-rich wacke in sandstone–mudstone couplets ranging from thin to very thick. Sandstone-rich intervals alternate with sandstone-poor packages of graded siltstone and mudstone, together with laminated siltstone like that below. Graptolites in the finer facies place the succession in the Homerian lundgreni Biozone.

Opposed current indicators. In the local account, flute and groove moulds suggest flow towards the north to north-east, whereas ripple cross-lamination indicates south to south-east flow. Reflection of turbidity currents has been proposed to explain the contrasting indicators. These local observations should remain distinct from differently directed regional dispersal models.

Fine-grained upper interval: regional context. The top of Birk Riggs includes a hemipelagic mud-rich interval described as 21–27 m across the district, or about 25 m along part of the outcrop in the later framework. An independently measured thickness for this exact local stream reach is not established.

Coldwell Siltstone Formation

A 30 m local section at Birk Riggs. The described exposure east of Birk Riggs, SD 2808 9536–2814 9529, is approximately 30 m thick. Larger district and regional ranges, 55–83 m and 47–100 m respectively, refer to different geographical scopes. Birk Riggs is now treated as a reference section, while Cold Well Quarry remains the type. The former Randy Pike and High Cross member names are retained below only as historical labels; the later framework recommends discontinuing their formal member status.

Lower calcareous interval, formerly Randy Pike Member. Blue-grey, bioturbated calcareous siltstone contains the trilobite Delops obtusicaudatus, the bivalve Slava interrupta, crinoid ossicles, brachiopod fragments and straight-shelled nautiloids. Laminated interbeds yield nassa Biozone graptolites. These Silurian shell-bearing beds are separate from the Ordovician fossil beds in Ashgill Quarry.

Middle laminated interval. Dark grey laminated siltstone separates the two calcareous packages and also yields nassa graptolites. Its individual local thickness is not established in the source used here; regional subdivision measurements cannot simply be fitted into the local 30 m total.

Horizon below the upper calcareous interval. Graptolites immediately below the former High Cross interval indicate the ludensis Biozone, a useful marker within the upper Wenlock part of the local sequence. No separate bed thickness is given.

Upper calcareous interval, formerly High Cross Member. Pale calcareous siltstone contains appreciable laminated interbeds and a sparser shelly fauna than the lower package. This interval reaches from the latest Wenlock into the earliest Ludlow; the historic member name does not imply a currently accepted formal rank.

The nilssoni horizon, 3 m below the top. Graptolites in a laminated siltstone show that the uppermost Coldwell succession extends into the Ludlow. The 3 m is the distance below the formation top, not the thickness of the fossil-bearing bed.

Wray Castle Mudstone Formation

Younger laminated mudrocks, 300 m locally. The local succession contains laminated siltstone with subordinate graded mudstone beds and nilssoni Biozone graptolites. It is a distributed geological section above Coldwell, not a 300 m quarry face or a thickness inferred from spoil.

CONISTON GROUP

The younger sandstone succession. Above Wray Castle, Ludlow sandstone-rich strata mark the transition into the Coniston Group. No complete local measured group log is established in these accounts. Regional totals approaching two kilometres do not describe a continuous visitor exposure.

Ashgill Quarry and Beck: the separate structural setting. The Ordovician quarry and adjacent beck succession dips south-east at about 30 degrees. Cleavage is especially conspicuous in its upper beds. This published local dip is distinct from the steeper bedding described in the younger Torver–Ashgill stream succession.

Structure and fossil provenance. In the Torver–Ashgill succession, beds generally strike north-east–south-west and dip south-east at about 40–60 degrees, while cleavage dips north-west at roughly 60 degrees. The Park Gill Thrust and steeper faults locally disturb the sequence. A cleavage surface is not necessarily a bedding plane, and fossils can be distorted. No single modern count confirms the old claim of more than 30 graptolite species throughout all the guide’s collecting places.

Skelgill detachment near Torver Beck and Timley Knott. Many north-west–south-east wrench faults turn into bedding-parallel thrusts within the softer Skelgill mudstones. Their mapped traces therefore do not simply end at the Ordovician–Silurian boundary. This structural detachment helps explain the variable Skelgill outcrop in this part of the protected area.

Type fossils from the wider named localities. The Torver form now called Tretaspis hadelandica deliqua was described from a holotype between Torver Beck and its first tributary. Torver material also contributed to the description of Primaspis bucculenta, but that species’ holotype is from Willy Scrow. A historically labelled Coniston Waterhead type of Paraharpes whittingtoni has uncertain provenance farther east and is not evidence of a discovery at Ashgill Quarry or Torver Beck.

References.
Owen and Rushton (1999), Ashgill Quarry, British Cambrian to Ordovician Stratigraphy, pp. 272–275.
Woodcock (2000), Torver–Ashgill, British Silurian Stratigraphy.
Millward and colleagues (2000), Geology of the Ambleside district, chapter 7.
Millward and Stone (2012), BGS framework for the Ordovician and Silurian sedimentary successions.
McNamara (1979), Lake District Ordovician trilobites.
Owen (1980), revision of Tretaspis.
Natural England: Ashgill Quarry SSSI.