Crag Farm Pit - Fossil Collecting Guide (UK Fossils)

Crag Farm Pit at Sudbourne is the type locality of the Sudbourne Member of the Coralline Crag. Cross-bedded shelly sand and bryozoans preserve evidence of Pliocene tidal sandwaves. The pit is on private land: arrange permission and confirm the available exposure before travelling, with no assumption of public entry or digging rights.

FIND FREQUENCY: ♦♦♦ – Documented bryozoans and shells; permitted collecting and exposure must be confirmed in advance.
CHILDREN: ♦♦ – Only within an authorised visit whose organiser considers the conditions suitable.
ACCESS: ♦♦♦ – Prior permission is required for entry, parking, approach and any limited collecting.
TYPE: – Private inland Coralline Crag pit and geological SSSI.

DIRECTIONS

♦ Arrange permission and confirm whether an exposure is currently available before travelling. Crag Farm lies east to south-east of Sudbourne, reached by the eastern road out of the village and the lanes leading to Crag Farm Road. The original route turned right at the first junction and left at the next towards the farm; use the landowner’s agreed approach and parking instructions rather than entering the farmyard unannounced. The historical west pit is at TM 4280 5232 and a second pit lay about 100 m farther east. The original guide’s reference is 52.11602°N, 1.54558°E. Neither historic mapping nor the SSSI boundary confirms that a pit is open for a visit today.

FOSSIL HUNTING

Crag Farm is particularly significant for bryozoans preserved in cross-bedded Coralline Crag. The leaf-like colonies traditionally described as Eschara pertusa are now treated as Pentapora pertusa; published revised material includes specimens specifically from Crag Farm, Sudbourne. Abraded pieces of Meandropora and Blumenbachium also occur, while calcitic scallops such as Aequipecten opercularis survive better than most aragonitic molluscs.

Large intact colonies are scientifically useful because they help distinguish growth near the site from transported shell debris. This is a description of documented material, not a guarantee of a presently accessible collecting face. If an authorised visit permits collecting, keep it to the small amount and loose material agreed with the owner; do not dig, carve specimens from the face or break up the section.

The original locality account also reports rounded colonies under the name Meandropora tubipora, with complete examples described as roughly tennis-ball sized. Broken specimens can reveal their internal structure. This is a historical local identification and size observation; do not damage a colony or the exposure to obtain a section.

A chronological record of geological research and significant discoveries at Crag Farm Pit, Sudbourne.

1841 – Pit mapped
The tithe map showed a substantial crag excavation west of the farm.

1898 – Harmer’s locality 18
Harmer included the western quarry in his survey of Coralline Crag localities.

1953 – Section reopened
Spencer later recorded the reopening of a long quarry section in this year.

1993 – Sudbourne Member type succession established
Balson and colleagues combined the pit with the research borehole in their lithostratigraphic subdivision of the Coralline Crag.

2010 – Modern bryozoan taxonomy
The systematic revision of Pentapora included Crag Farm museum specimens of Pentapora pertusa, clarifying the historical name used in older accounts.

2022–2023 – Exposures confirmed in favourable condition
Natural England’s assessment published in March 2023, following its April 2022 field visit, records well-exposed cross-bedding, bryozoan-rich beds and fissure fills. The assessment describes the geology, not a public right of access.

References
Balson 1999: Crag Farm, GCR volume 15, pp. 277–281
BGS Woodbridge/Felixstowe explanation: composite stratotype
Taylor and Milne: Coralline Crag bryozoans
Lombardi and colleagues 2010: Pentapora revision
Natural England: Crag Farm Pit, Sudbourne SSSI
Natural England: unit condition, March 2023 assessment

GEOLOGY

Crag Farm is the type locality of the Sudbourne Member of the Pliocene Coralline Crag Formation. Its sediment is a carbonate-rich sand made largely from fragments of shells and bryozoan skeletons. The striking inclined layers are the preserved slopes of underwater sandwaves, with predominant south-west dips recording their direction of migration. Coarser bryozoan fragments became concentrated towards the bases of sets and the lower parts of these slopes. The pit therefore records the movement and sorting of skeletal debris through a shallow sea, rather than simply a mass of animals preserved where they lived.

Not all the fossils were transported equally. Some well-preserved Pentapora pertusa colonies have been interpreted as growth during pauses in sand movement, followed by burial, whereas more abraded fragments indicate reworking. Later groundwater alteration further changed the assemblage. Much of the exposed sand lost its aragonitic shell material through dissolution, leaving more resistant calcite skeletons; calcite also partly cemented the sediment. Joints cutting the beds contain younger sediment and later calcite infill, showing that the rock continued to change long after the sandwaves stopped moving.

The fuller succession is known from a research borehole as well as the pit. Beneath the floor, lower Sudbourne sediment retains aragonite, and the underlying Ramsholt Member contains silty shelly sands above London Clay. These underground beds cannot be treated as additional exposed collecting horizons. Natural England’s April 2022 visit, reported in March 2023, confirmed excellent exposed sedimentary structures; the published composite thickness nevertheless combines the historical face with the borehole rather than describing a single visible wall.

Detailed stratigraphy and geological evidence at Crag Farm Pit, Sudbourne.

CRAG GROUP

The marine Coralline Crag here predates the younger shelly Red Crag elsewhere in Suffolk.

Coralline Crag Formation

The composite type section combines a historic pit face and a borehole; its total depth is not the height of a visitor-accessible face.

Sudbourne Member

The GCR account recorded 5.6 m above the pit floor, with a further 6.3 m below it, giving 11.9 m in the composite section. The exposed calcarenite was leached of aragonite, with partial calcite cementation. The sand fraction is medium-grained and the skeletal calcarenite contains less than 5% mud. Figure 10.17 records retained aragonite in the lowermost 2 m of the Sudbourne Member, beneath the pit floor; dissolution therefore did not affect the whole composite succession equally.

Cross-bedded sets and bottomsets

Observed sets were commonly 0.5–2 m thick; the composite log shows sets up to 2.5 m. Foresets dip predominantly south-west, with broad troughs in faces cut across the transport direction. Coarse bryozoan fragments concentrate in bottomsets and lower foresets. Thin silty layers and occasional graded cycles survive, but tidal-rhythm interpretation is less clear than in the Red Crag.

Fossil-bearing foresets

Some well-preserved Pentapora pertusa colonies were interpreted as growth during pauses in sand movement, followed by burial. The transported assemblage is more abraded. Vertical lined Skolithos and sinuous unlined Planolites occur as trace fossils.

Joints, infill and late calcite

Orthogonal joints cut the beds. Some contain younger shelly sediment, historically considered probably early Pleistocene, and white micritic calcite with root-related structures. Calcite formation postdates both aragonite dissolution and the sedimentary fissure filling.

Ramsholt Member — below the pit floor

The borehole proved 5.9 m of silty shelly sands with preserved aragonitic shells and glauconite. Phosphatic mudstone pebbles occur towards its base. These beds lie beneath the pit floor, rather than in the exposed quarry face.

THAMES GROUP — BURIED SUBSTRATE

This separate older group lies beneath the Crag succession and is known here from subsurface evidence.

London Clay Formation — borehole substrate

Micaceous silty clay was reached below the Coralline Crag, at approximately −10.4 m OD. It was not exposed in the documented pit face.

References
UK Fossils: Crag Farm Pit, original route and historic restrictions
Balson 1999: Crag Farm, GCR volume 15, pp. 277–281
BGS Woodbridge/Felixstowe explanation: composite stratotype
Taylor and Milne: Coralline Crag bryozoans
Lombardi and colleagues 2010: Pentapora revision
Natural England: Crag Farm Pit, Sudbourne SSSI
Natural England: unit condition, March 2023 assessment

SAFETY

Any permitted visit must follow the owner’s current assessment of the ground. Former quarry faces may be unstable or overhanging, while infilled or vegetated areas can hide edges and uneven surfaces. Stay away from faces and rims, do not climb or undercut, and keep clear of farm machinery and working areas. If the owner cannot provide a safe authorised viewing place, do not enter.

EQUIPMENT

A camera, hand lens, sturdy boots and small padded boxes are sufficient for an agreed observational visit and any permitted loose finds. Do not use chisels, picks or other tools to excavate this no-digging site.

CLEANING AND TREATING

Keep each permitted specimen with its locality and bed information. Support fragile shells or bryozoans in tissue inside a rigid box. Allow damp specimens to dry slowly away from heat, then begin with a soft brush. Do not force compacted sand from delicate shell edges. Avoid acids, bleach and varnish. If an exceptionally fragile specimen needs consolidation, seek advice before using a reversible conservation adhesive such as Paraloid B-72; routine coating is unnecessary.

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ACCESS RIGHTS

This is private land. Arrange permission from the landowner or operator before entering the pit, including agreement on the approach, parking and any collecting. A public path nearby does not give permission to enter the excavation. Follow the visit organiser’s conditions; permission for a visit is not automatically permission to dig, clear vegetation or take bulk samples. Crag Farm Pit, Sudbourne is a geological SSSI. SSSI status does not grant public access or collecting rights. The original guide specified no digging and only very limited specimen removal; confirm all current conditions with the owner before visiting.

It is important to follow our ‘Code of Conduct’ when collecting fossils or visiting any site. Please also read our ‘Terms and Conditions’.

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