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RESEARCH ARTICLE (Open Access)

The seafloor character of the Roman Rock area in False Bay, South Africa

Andrew Terhorst https://orcid.org/0000-0002-0955-5251 A * and John Rogers B
+ Author Affiliations
- Author Affiliations

A Commonwealth Scientific and Industrial Research Organisation, Hobart, Tas., Australia.

B Department of Geology, University of Cape Town, Rondebosch, South Africa.

* Correspondence to: andrew.terhorst@csiro.au

Handling Editor: Daniel Deng

Marine and Freshwater Research 76, MF24220 https://doi.org/10.1071/MF24220
Submitted: 11 October 2024  Accepted: 11 August 2025  Published: 5 September 2025

© 2025 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)

Abstract

Context

This study investigates the seafloor character of a 12-km2 area off Simon’s Town in False Bay, South Africa.

Aims

The goal was to map the seafloor geology around Roman Rock Lighthouse, focusing on sedimentology.

Methods

Side-scan sonar and single-beam echosounder were used, complemented by sediment sampling and diver observations for ground-truthing. Sediment samples were analysed for texture and composition.

Key results

Six patterns of reflectivity (acoustic facies) were identified, including granite outcrops, wave ripples, and patches of gravelly and quartzose sands. Sediment analysis showed a mix of calcareous and quartzose components, with calcareous material dominating the gravel fraction near granite outcrops.

Conclusions

The Roman Rock seafloor reflects a complex depositional environment shaped by biogenic activity and wave-induced sediment transport.

Implications

This research lays the groundwork for future geological studies in False Bay, particularly around wave-induced sediment dynamics.

Keywords: diver observations, echosounder, False Bay, marine geology, Roman Rock, seafloor mapping, sedimentology, side-scan sonar, South Africa.

References

Atkins GR (1970) Winds and current patterns in False Bay. Transactions of the Royal Society of South Africa 39(2), 139-148.
| Crossref | Google Scholar |

Backeberg NR, Reid DL, Trumbull RB, Romer RL (2011) Petrogenesis of the False Bay Dyke Swarm, Cape Peninsula, South Africa: evidence for basement assimilation. South African Journal of Geology 114(3-4), 335-352.
| Crossref | Google Scholar |

Bagnold RA (1963) Mechanics of marine sedimentation. In ‘The sea: ideas and observations. Vol. 3’. (Eds MN Hill, ED Goldberg, CO Iselin, WH Munk) pp. 507–528. (Interscience: New York, NY, USA)

Bang ND (1967) Oceanography and a naval tragedy. Lantern 17(1), 84-87.
| Google Scholar |

Barwis JH, Tankard JA (1983) Pleistocene shoreline deposition and sea-level history at Swartklip, South Africa. SEPM Journal of Sedimentary Research 53(4), 1281-1294.
| Crossref | Google Scholar |

Belcher RW, Kisters AFM (2003) Lithostratigraphic correlations in the western branch of the Pan-African Saldania belt, South Africa: the Malmesbury Group revisited. South African Journal of Geology 106(4), 327-342.
| Crossref | Google Scholar |

Birch GF (1981) The karbonat-bombe: a precise, rapid and cheap instrument for determining calcium carbonate in sediments and rocks. Transactions of the Geological Society of South Africa 84(3), 199-203.
| Google Scholar |

Boocock C (1951) The structural features and inclusions of the Cape Peninsula granite. Transactions of the Royal Society of South Africa 33(2), 243-276.
| Crossref | Google Scholar |

Bosence DWJ (1976) Ecological studies on two unattached coralline algae from western Ireland. Palaeontology 19(2), 365-395.
| Google Scholar |

Bouma AH, Rappeport ML (1984) Verification of side-scan sonar acoustic imagery by underwater photography. In ‘Underwater photography’. (Ed. PL Smith) pp. 279–294. (Van Nostrand Reinhold)

Bowie DK (1966) The marine geology of False Bay. MSc thesis, University of Cape Town, Cape Town, South Africa.

Branch G, Branch M (1981) ‘The living shores of southern Africa.’ (Struik Publishers)

Brink VDS, Rogers J (1985) Rapid granulometry of sand using a computer-linked settling-tube. Bulletin 15, Joint Geological Survey–University of Cape Town Marine Geology Programme, Cape Town, South Africa.

Brundrit G (2009) Global climate change and adaptation: City of Cape Town sea-level rise risk assessment. Econlogic, Cape Town, South Africa.

Caldwell PC, Merrifield MA, Thompson PR (2015) Sea level measured by tide gauges from global oceans – the Joint Archive for Sea Level holdings (NCEI Accession 0019568), Version 5.5. NOAA National Centers for Environmental Information, Hawai’i, USA.

Clark PU, Dyke AS, Shakun JD, Carlson AE, Clark J, Wohlfarth B, Mitrovica JX, Hostetler SW, McCabe AM (2009) The last glacial maximum. Science 325(5941), 710-714.
| Crossref | Google Scholar | PubMed |

Coleman F, Diedericks GPJ, Theron AK, Lencart e Silva J (2021) Three-dimensional modelling of the circulation in False Bay, South Africa. African Journal of Marine Science 43(1), 95-118.
| Crossref | Google Scholar |

Compton JS, Mulabisana J, McMillan IK (2002) Origin and age of phosphorite from the Last Glacial Maximum to Holocene transgressive succession off the Orange River, South Africa. Marine Geology 186(3-4), 243-261.
| Crossref | Google Scholar |

Cooper JAG, Green AN, Compton JS (2018) Sea-level change in southern Africa since the Last Glacial Maximum. Quaternary Science Reviews 201, 303-318.
| Crossref | Google Scholar |

Daniels T, Fearon G, Vilaplana A, Hewitson B, Rautenbach C (2022) On the importance of wind generated waves in embayments with complex orographic features – a South African case study. Applied Ocean Research 128, 103355.
| Crossref | Google Scholar |

Darbyshire M (1966) The surface waters near the coasts of Southern Africa. Deep Sea Research and Oceanographic Abstracts 13, 57-81.
| Crossref | Google Scholar |

Davies JL (1980) ‘Geographical variation in coastal development. Geomorphology Texts’, 4th edn. (Longman)

Day JH (1969) ‘A guide to the marine life on South African shores.’ (Balkema)

De Jong CD, Elema IA, Lachapelle G, Skone S (2002) ‘Hydrography.’ (DUP Blue Print)

De La Cruz MA (1978) Marine geophysical and geological investigations in Saldanha Bay. Bulletin 9, Joint Geological Survey–University of Cape Town Marine Geology Programme, Cape Town, South Africa.

de Vos M, Vichi M, Rautenbach C (2021) Simulating the coastal ocean circulation near the Cape Peninsula using a coupled numerical model. Journal of Marine Science and Engineering 9(4), 359.
| Crossref | Google Scholar |

Du Plessis A, Glass JG (1991) The geology of False Bay. Transactions of the Royal Society of South Africa 47, 495-517.
| Google Scholar |

Dube LT (2002) Climate of Southern Africa. South African Geographical Journal 84(1), 125-138.
| Crossref | Google Scholar |

Duck RW, McManus J (1985) A sidescan sonar survey of a previously drawn-down reservoir: a control experiment. International Journal of Remote Sensing 6(5), 601-609.
| Crossref | Google Scholar |

Flemming BW (1976a) Rocky Bank – evidence for a relict wave-cut platform. Annals of the South African Museum 71, 33-48.
| Google Scholar |

Flemming BW (1976b) Side-scan sonar: a practical guide. The International Hydrographic Review 53, 65-92.
| Google Scholar |

Flemming BW (1982) The geology of False Bay (Western Cape, South Africa) with special emphasis on modern sediments. National Research Institute for Oceanology Report C/Sea 8253, CSIR, Stellenbosch, South Africa.

Flemming BW (2019) Ripples and dunes: do flumes tell the whole story? In ‘MARID VI. Marine and River Dune Dynamics 2019. Book of Abstracts’, 1–3 April 2019, Bremen, Germany. (Eds A Lefebvre, T Garlan, C Winter) pp. 89–94. (Center for Marine Environmental Sciences, University Bremen and SHOM) Available at https://www.marum.de/Binaries/Binary18548/MARIDVI-Books-of-proceedings.pdf

Flemming BW (2024) Sedimentology of False Bay (Western Cape, South Africa): geological background information for the integrated environmental management of a physically confined coastal compartment. Journal of Coastal Conservation 28(6), 78.
| Crossref | Google Scholar |

Flemming BW, Thum AB (1978) The settling tube – a hydraulic method for grain size analysis of sands. Kieler Meeresforschungen Sonderheft 4, 82-95.
| Google Scholar |

Folk RL, Ward WC (1957) Brazos River bar [Texas]; a study in the significance of grain size parameters. Journal of Sedimentary Research 27(1), 3-26.
| Crossref | Google Scholar |

Foster MS (2001) Rhodoliths: between rocks and soft places. Journal of Phycology 37(5), 659-667.
| Crossref | Google Scholar |

Fourie JP, Ansorge I, Backeberg B, Cawthra HC, MacHutchon MR, van Zyl FW (2015) The influence of wave action on coastal erosion along Monwabisi Beach, Cape Town. South African Journal of Geomatics 4(2), 96-109.
| Crossref | Google Scholar |

Gentle RI (1971) Pre-Quaternary geology of the continental shelf between Cape Infanta and Cape Town. Technical Report, Marine Geology Programme 3, pp. 13–27, South African National Commission of Oceanographic Research.

Gentle RI (1973) Sidescan sonar in marine geological investigations of the South African continental shelf. South African Journal of Science 69(12), 360-367.
| Google Scholar |

Gerlach RW, Dobb DE, Raab GA, Nocerino JM (2002) Gy Sampling theory in environmental studies. 1. Assessing soil splitting protocols. Journal of Chemometrics 16(7), 321-328.
| Crossref | Google Scholar |

Glass JGK (1977) Deep weathering of the southwestern Cape Granite and Malmesbury Group: palaeoclimatic implications. In ‘Technical Report Number 9’. pp. 118–135. (Joint Geological Survey–University of Cape Town Marine Geosciences Group)

Glass G (1980) Geology, morphology, sediment cover and movement. In ‘The future management of False Bay’. (Ed. B Gasson) pp. 15–25. (False Bay Conservation Society: Cape Town, South Africa)

Grundlingh ML, Largier JL (1991) Physical oceanography of False Bay: a review. Transactions of the Royal Society of South Africa 47(4–5), 387-400.
| Google Scholar |

Haughton SH (1933) The geology of Cape Town and adjoining country. Explanation accompanying the geological map of Cape Town and adjoining country, Geological Survey, South Africa.

Hay ER (1981) A stratigraphic and sedimentological analysis of borehole data from a portion of the cape flats. BSc(Hons) thesis, University of Cape Town, Cape Town, South Africa.

Hersbach H, Bell B, Berrisford P, Hirahara S, Horányi A, Muñoz-Sabater J, Nicolas J, et al. (2020) The ERA5 Global Reanalysis. Quarterly Journal of the Royal Meteorological Society 146(730), 1999-2049.
| Crossref | Google Scholar |

Ingram RL (1965) Facies maps based on the megascopic examination of modern sediments. Journal of Sedimentary Research 35(3), 619-625.
| Crossref | Google Scholar |

James NP, Bone Y, Von Der Borch CC, Gostin VA (1992) Modern carbonate and terrigenous clastic sediments on a cool water, high energy, mid-latitude shelf: Lacepede, southern Australia. Sedimentology 39(5), 877-903.
| Crossref | Google Scholar |

Johansen HW (2018) ‘Coralline algae: a first synthesis.’ (CRC Press)

Jury MR (2020) Coastal gradients in False Bay, south of Cape Town: what insights can be gained from mesoscale reanalysis? Ocean Science 16(6), 1545-1557.
| Crossref | Google Scholar |

Kidd RB, Simm RW, Searle RC (1985) Sonar acoustic facies and sediment distribution on an area of the deep ocean floor. Marine and Petroleum Geology 2(3), 210-221.
| Crossref | Google Scholar |

Kilburn R, Rippey E (1982) ‘Sea shells of southern Africa.’ (Macmillan: Cape Town, South Africa)

Kistner S (2016) Monwabisi: a hydrodynamic study of the hazardous cell circulation and potential related solutions to a safer bathing facility. MEng thesis, Stellenbosch University, Stellenbosch, South Africa.

Komar PD, Miller MC (1973) The threshold of sediment movement under oscillatory water waves. Journal of Sedimentary Petrology 43(4), 1101-1110.
| Google Scholar |

Komar PD, Miller CM (1975) On the comparison between the threshold of sediment motion under waves and unidirectional currents with a discussion of the practical evaluation of the threshold: reply. SEPM Journal of Sedimentary Research 45(1), 362-367.
| Crossref | Google Scholar |

Li Y, Yu Q, Gao S, Flemming BW (2020) Settling velocity and drag coefficient of platy shell fragments. Sedimentology 67(4), 2095-2110.
| Crossref | Google Scholar |

Linton DL (1955) The problem of tors. The Geographical Journal 121(4), 470-487.
| Crossref | Google Scholar |

MacHutchon M (2015) Geophysical monitoring of coastal erosion and cliff retreat of Monwabisi Beach, False Bay, South Africa. South African Journal of Geomatics 4(2), 80-95.
| Crossref | Google Scholar |

Mallory JK (1970) The bathymetry and microrelief of False Bay. Transactions of the Royal Society of South Africa 39(2), 109-112.
| Google Scholar |

Manikam SL, Green AN, Cooper A, Deacon G, Flemming B (2022) Development and preservation of transgressive sandy versus rocky shorelines: observations from the SE African Shelf. Geomorphology 419, 108485.
| Crossref | Google Scholar |

Manikam SL, Green AN, Flemming BW, Cooper A, Deacon G (2024) Bedform characteristics and implications for seafloor–bottom current interactions along the Wild Coast Shelf, South Africa. Continental Shelf Research 272, 105159.
| Crossref | Google Scholar |

Martins LR, Martins IR, Urien CM (2005) Sand Bodies of the Santa Catarina Inner Continental Shelf, Brazil. Gravel 3, 103-108.
| Google Scholar |

McManus DA (1975) Modern versus relict sediment on the continental shelf. Geological Society of America Bulletin 86(8), 1154-1160.
| Crossref | Google Scholar |

Morgans JFC (1956) The benthic ecology of False Bay, with notes on the analysis of shallow-water soft substrata. PhD thesis, University of Cape Town, Cape Town, South Africa.

Mount JF (1984) Mixing of siliciclastic and carbonate sediments in shallow shelf environments. Geology 12(7), 432-435.
| Crossref | Google Scholar |

Müller G, Gastner M (1971) The ‘Karbonat-Bombe’, a simple device for the determination of carbonate content in sediment, soils, and other materials. Neues Jahrbuch Für Mineralogie-Monatshefte 10, 466-469.
| Google Scholar |

Munsell A (1975) ‘Soil color charts.’ (Macbeth Division of Kollmorgen Corporation: Baltimore, MD, USA)

Pfaff MC, Logston RC, Raemaekers SJPN, Hermes JC, Blamey LK, Cawthra HC, Colenbrander DR, et al. (2019) A synthesis of three decades of socio-ecological change in False Bay, South Africa: setting the scene for multidisciplinary research and management. Elementa: Science of the Anthropocene 7, 32.
| Crossref | Google Scholar |

Rautenbach C, Barnes MA, de Vos M (2019) Tidal characteristics of South Africa. Deep-Sea Research – I. Oceanographic Research Papers 150, 103079.
| Crossref | Google Scholar |

Reineck H-E, Singh IB (1980) Current and wave ripples. In ‘Depositional sedimentary environments’. (Eds H-E Reineck, IB Singh) pp. 22–55. (Springer)

Retief GF (1970) Sediment transport in Gordon’s Bay. Transactions of the Royal Society of South Africa 39(2), 163-182.
| Crossref | Google Scholar |

Rogers J (2018) ‘Geological adventures in the fairest cape: unlocking the secrets of its scenery.’ (Council for Geoscience Pretoria)

Russell-Cargill WGA (1983) Project Hydra: general background to the project development. Technical Report Tv-019-82, Institute for Maritime Technology, Simon’s Town, South Africa.

Salonen N, Rautenbach C (2021) Toward nearshore, bathymetry induced wave amplification in False Bay, South Africa. AIP Advances 11(7), 075209.
| Crossref | Google Scholar |

Scheepers R, Schoch AE (2006) The Cape Granite Suite. In ‘The geology of South Africa’. (Eds MR Johnson, CR Anhaeusser, RJ Thomas) pp. 421–432. (Geological Society of South Africa)

Schoonees JS, Scholtz DJP, Van Tonder A, Moller JP, Lenhoff L (1983) Valsbaai: Veldata verslag. Tech. Report C/Sea 8219, CSIR, South Africa. [In Afrikaans]

Schulze BR (1965) Climate of South Africa. Part 8, General Survey. SA Weather Bureau.

Shipley AM (1964) Some aspects of wave refraction in False Bay. South African Journal of Science 60(4), 115-120.
| Google Scholar |

Simpson ESW, du Plessis A, Forder E (1970) Bathymetric and magnetic traverse measurements in False Bay and west of the Cape Peninsula. Transactions of the Royal Society of South Africa 39(2), 113-116.
| Crossref | Google Scholar |

Steller DL, Riosmena-Rodriguez R, Foster MS, Roberts CA (2003) Rhodolith bed diversity in the Gulf of California: the importance of rhodolith structure and consequences of disturbance. Aquatic Conservation: Marine and Freshwater Ecosystems 13(S1), S5-s20.
| Crossref | Google Scholar |

Steneck RS (1986) The ecology of coralline algal crusts: convergent patterns and adaptative strategies. Annual Review of Ecology and Systematics 17(1), 273-303.
| Crossref | Google Scholar |

Stubblefield WL, Mcgrail DW, Kersey DG (1984) Recognition of transgressive and post-transgressive sand ridges on the New Jersey continental shelf. In ‘Siliciclastic Shelf Sediments’. (Eds RW Tillman, CT Siemers) pp. 2–23. (SEPM Society for Sedimentary Geology) doi:10.2110/csp.99.07.0053

Swift DJP, Stanley DJ, Curray JR (1971) Relict sediments on continental shelves: a reconsideration. The Journal of Geology 79(3), 322-346.
| Crossref | Google Scholar |

Swift DJP, Parker G, Lanfredi NW, Perillo G, Figge K (1978) Shoreface-connected sand ridges on American and European shelves: a comparison. Estuarine and Coastal Marine Science 7(3), 257-273.
| Crossref | Google Scholar |

Terhorst AL (1988) The seafloor environment off Simon’s Town in False Bay, revealed by side-scan sonar, bottom sampling, diver observations and underwater photography. Bulletin 22, Joint Geological Survey–University of Cape Town Marine Geoscience Unit, Cape Town, South Africa.

Terhorst A, Rogers J (2025) Seafloor character of the Roman Rock area in False Bay, South Africa. EarthArXiv 2025, version 3 [Preprint, published 27 February 2025].
| Crossref | Google Scholar |

Theron JN (1984) ‘The geology of Cape Town and environs: Explanation sheets for 3318 CD and DC and 3418 AB, AD and BA.’ (Geological Survey of South Africa)

Theron AK, Schoonees JS (2007) Sand transport at and shoreline response to a breakwater attached to a large tidal pool at Monwabisi, Cape Town. Journal of the South African Institution of Civil Engineering 49(2), 2-9.
| Google Scholar |

Van Veen J (1936) Onderzoekingen in de Hoofden in Verband Met de Gesteldheid Der Nederlandsche Kust. Technical Report, Rijkswaterstaat, Netherlands. [In Dutch]

Van Zyl FW (2011) Marine geophysical survey report: table mountain marine protected area phase 2 (False Bay–Simons Town to Cape Point). Council for Geoscience, Bellville, Cape Town, South Africa.

Wainman CK, Polito A, Nelson G (1987) Winds and subsurface currents in the False Bay region, South Africa. South African Journal of Marine Science 5(1), 337-346.
| Crossref | Google Scholar |

Woelkerling WJ, Irvine LM, Harvey AS (1993) Growth-forms in non-geniculate coralline red algae (Coralliinales, Rhodophyta). Australian Systematic Botany 6(4), 277-293.
| Crossref | Google Scholar |

Yokoyama Y, Lambeck K, De Deckker P, Johnston P, Fifield LK (2000) Timing of the Last Glacial Maximum from observed sea-level minima. Nature 406(6797), 713-716.
| Crossref | Google Scholar | PubMed |