Fremington Clay Erratics: Elevation and Stratigraphic Context

The Fremington Clay of North Devon has long been recognised as one of the few possible glacial deposits in South West England. Reports of erratics and dropstones have played an important role in interpreting its depositional environment as glacilacustrine. However, the precise stratigraphic context and elevation (OD, above sea level) of these erratics requires clarification.

This document synthesises evidence from published studies (Hawkins & Hawkins, 1990; Cattell, 2003; Scourse, 2024; GeoGuide/GCR entries), historical reports, and BGS memoirs to establish the depth, stratigraphic position, and elevation range of the erratic-bearing horizons. Additional historical detail is incorporated from Arber (1964), which provides first-hand accounts from local workers confirming the embedded positions of erratics within the clay sequence.

1. Historical Observations from Clay Pits

• 19th–20th century clay pits near Fremington and Roundswell exposed the Fremington Clay. Erratics of sandstone, quartzite, granite, dolerite, and other lithologies were recorded (Dewey, 1910; Taylor, 1956; Maw, 1864; Arber, 1964).

• Higher Gorse (Brannam’s) Clay Pit: BGS (1970) log shows:

  • 4 m of stony/boulder clay (erratic-bearing)
  • 6 m of largely stone-free lacustrine clay ("pottery clay")
  • Underlain by basal gravels (2 m thick)

• These pits sit at low topographic elevations (~10–20 m OD).

• Detailed stratigraphic insights from Arber (1964), based on local informants and pit observations at Bickington (Tews Lane pits) and nearby farms:

  • General sequence: Basal gravel over Culm Measures, followed by clay with stones, smooth brown potting clay (up to 21 feet/6.4 m thick), "horseflesh" clay with small stones, grit, and decayed wood (2+ feet/0.6+ m), and surface gravel (up to 16+ feet/4.9+ m thick, also erratic-bearing).
  • Specific erratic positions:
    • Combrew Farm: Vesicular granophyre (erratic No. 6 in Taylor, 1956) found isolated in the middle of the clay bed.
    • Chilcotts Farm: Hypersthene-andesite (erratic No. 7 in Taylor, 1956) excavated ~22 feet (6.7 m) below ground surface around 1870.
    • Bickington pits: Quartz-dolerite boulder (No. 9 in Taylor, 1956) in the very middle of the brown clay; another smooth rounded boulder (unidentified, ~19 inches/48 cm long) 10 feet (3 m) below the upper clay surface; another 16 feet (4.9 m) below the upper clay surface; another quartz-dolerite (No. 13 in Taylor, 1956) ~10 feet (3 m) from the clay top.
    • Over 50 small pebbles collected since 1956, mostly at or near the base or top of the clay, but some embedded 5–11 feet (1.5–3.4 m) above the base. Two pebbles (one olivine-dolerite) found 2–3 feet (0.6–0.9 m) above the base in 1955.

• Arber (1964) emphasises that these observations provide direct evidence of erratics embedded in the "heart" of the Fremington Clay, reinforcing its glacial origin (e.g., as bottom-moraine from an Irish Sea ice-sheet, per Zeuner, 1959).

References: Hawkins & Hawkins (1990); Edmonds et al. (1985); GeoGuide/GCR14 Brannam; Arber (1964).

2. Lake Cutting (Barnstaple Bypass)

• Excavation up to 9 m deep exposed a complete Fremington Clay sequence (Hawkins & Hawkins, 1990).

• Stratigraphy:

  • Unit A: gravelly clay with many clasts (interpreted as till/head)
  • Unit B: stony clay with fewer clasts (erratic-bearing)
  • Unit C: red, laminated silty clay, stone-free (glacilacustrine)
  • Unit D: thin fine sand
  • Unit E: coarse glaciofluvial sandy gravel over bedrock

• Erratic clasts (sandstone, mudstone, quartz, granite, dolerite) concentrated in Units A & B.

Elevation control (Cattell, 2003):

• Crest of cutting: ~30 m OD.
• Base of clays: dips from ~26 m OD (crest) to ~20 m OD (road level beneath Lake Overbridge).
• Boreholes north of the road confirm clay–gravel contact at 19.9–20.4 m OD.

Thus, the erratic-bearing horizons lie between ~20–26 m OD.

References: Hawkins & Hawkins (1990); Cattell (2003).

3. GeoGuide / GCR14 Brannam’s Clay Pit

• Fremington Clay thickness up to ~27 m.
• Sequence: basal gravels → stoneless laminated clays (glacilacustrine) → increasingly stony clays with occasional dropstones.
• One striated microdolerite cobble reported in situ within unit B (Croot et al., 1996).
• Confirms dropstones/erratics occur in upper clay units, but does not provide OD values. This aligns with Arber (1964)'s observations of erratics in the middle and upper parts of the smooth clay, transitioning to stony layers.

Reference: GeoGuide Scottish Geology Trust (GCR14 Brannam).

4. Regional Context (Scourse, 2024)

• Focuses on raised beaches and relative sea-level history (MIS 4–3).
• Confirms Fremington deposits interpreted as glacilacustrine with occasional dropstones.
• Does not provide new OD control for erratic horizons, but supports the glacial interpretation consistent with historical evidence like Arber (1964).

Reference: Scourse (2024, Journal of Quaternary Science).

5. Synthesis

• Erratic-bearing horizons (stony clays, Units A/B) occur in the upper Fremington Clay sequence, consistently recorded in both pits and the Lake cutting. Arber (1964) adds granular detail on embedded erratics at depths of 5–22 feet (1.5–6.7 m) within the clay, including the middle of the potting clay layer, corroborating the presence of dropstones throughout the deposit rather than solely at boundaries.

• Stone-free laminated clays (Unit C) represent the main lacustrine body (dropstone-poor).

• Elevation control:

  • Clay pits (Higher Gorse/Brannam, Bickington): near sea level, ~10–20 m OD, with erratics found at depths implying positions well below 20 m OD.
  • Lake cutting (Barnstaple bypass): erratics at ~20–26 m OD.

• No evidence supports claims that erratics occur at "quite high elevation"; exposures are confined to low ground just above sea level. The depths reported in Arber (1964) further indicate erratics are not surficial but integrated into the low-elevation clay body.

6: Conclusion:

At Fremington, exposures in clay pits and in the Lake cutting reveal a glacilacustrine clay sequence with occasional dropstones and erratic clasts embedded at various depths within the deposit. These horizons occur at low elevations, between ~10–26 m OD, rather than on high ground.

7: References:

Arber, M.A. (1964). Erratic boulders within the Fremington Clay of North Devon. Geological Magazine, 101(3), 282–283. https://doi.org/10.1017/S0016756800049517

Hawkins, A.B. & Hawkins, S.C. (1990). Quaternary deposits in the Lake Cutting of the Barnstaple Bypass, North Devon. Proceedings of the Ussher Society, 7, 301–303. http://ussher.org.uk/wp-content/uploads/journal/1990/21-Hawkins_Hawkins1990.pdf
Cattell, A. (2003). Geological and geotechnical aspects of a landslip in the Fremington Clay, North Devon. Geoscience in South-West England, 10, 397–402. http://ussher.org.uk/wp-content/uploads/journal/2003/03-Cattell_2003.pdf
Edmonds, E.A., Whittaker, A. & Williams, B.J. (1985). Geology of the country around Ilfracombe and Barnstaple. BGS Memoir. https://www.google.co.uk/books/edition/Geology_of_the_Country_Around_Ilfracombe/IxNXAAAAMAAJ?hl=en
Croot, D., Gilbert, A., Griffiths, J. & van der Meer, J.J. (1996). The character, age and depositional environments of the Fremington Clay Series, North Devon. In: Charman et al. (eds), Devon & East Cornwall Field Guide, Quaternary Research Association. https://researchportal.plymouth.ac.uk/en/publications/the-character-age-and-depositional-environments-of-the-fremington
GeoGuide Scottish Geology Trust (GCR14 Brannam’s Clay Pit): https://geoguide.scottishgeologytrust.org/p/gcr14/gcr14_brannam
Scourse, J. (2024). Pleistocene raised beaches of south-west Britain and the last glaciation of the Celtic Sea. Journal of Quaternary Science. DOI: 10.1002/jqs.3611