In the early 1960s I was introduced to debates about “beachrock” (sometimes spelt “beach rock”). I attempted to translate a paper in French by Andre Guilcher on the subject; this proved beyond me although it whetted the appetite to know more. A carbonate cemented rock exposed at Fishermans Beach near Collaroy had been noted by Edgeworth David in a report I found in the Mines Department library. This was about the time I left Sydney to undertake graduate studies at Louisiana State University. The head of the Coastal Studies Institute at LSU was Professor Richard J. Russell and I knew from the literature that he was undertaking research on beachrock. A sample was collected from Fishermans to show him. Well that turned out to be a frustrating experience.
Before recounting that experience I must go back a few steps. During World War II the US Marines sometimes found their landing craft aground on hard rock on Pacific islands where surveillance suggested soft sand beaches. The US Navy wanted to know more as did those preparing for post-war atomic tests. Geological studies by the US Geological Survey on atolls (e.g. Bikini) revealed much about this rock (see USGS Professional Paper 260-A, 1954). In the mid-1950s the Geography Branch of the Office of Naval Research (ONR) tasked Russell at LSU to undertake a global study starting in the Caribbean, and later around shores of the Mediterranean, Pacific and Indian Oceans. It included sites along both east and west coasts of Australia (e.g. at Fannie Bay, Darwin, which I visited in July 2023, see blog 244). Over a 10-year period he assembled a vast observational record coupled with analytical studies involving specialist colleagues in petrography and microbiology. The result was a series of publications, the last of which appeared in The Geographical Review in 1965 on Southern Hemisphere beachrock (vol LV, p.17-45, see figs 5 to 9 for Australian photos).
Russell developed very firm ideas on the origin of beachrock. He focused on what is sometimes referred to as “beach rock sensu stricto” or “pure” beachrock. This rock is differentiated from other types of carbonate cemented rocks of varying age that may appear on an active beachface. In his 1965 paper he defined this pure form as “beach material bonded by calcareous cement that is exposed by removal of covering material by waves”. He goes on to note that “The appearance of beach rock is distinctive. One or more bands (pavements) crop out about sea level if the tidal range is small or about high neap-tide level if it is large, and invariably at the level of the spring line where groundwater seeps out”. His views are also in earlier papers in the Zeitschrift fur Geomorphologie, one based on Caribbean observations (1959, vol. 3, p.227-236), the other on the origin of the material (1962, vol. 6, p.1-16). His work relied on his own field observations along with measures of water chemistry and temperature and some geochemical analyses by colleagues.
The role of fluctuating water table was central to Russell’s thinking. Cementation begins within this fluctuating zone not from dissolved calcium carbonate seeping downward through loose sand of the covering beach. On a retreating shore inclined older layers of beachrock lie above younger layers which penetrate landwards as “incipient” beachrock. His biological colleagues found no evidence for algae assisting the cementation process leading him to conclude that “cementation may result in a straightforward way from the evaporation of ground water through the interstices between sand grains”. Percolating sea water alone was not responsible for cementation around the migrating water table which may be diluted by freshwater. He concluded that “as beach rock is not strictly a marine feature, sea water temperature is not concerned in its initial cementation. The critical control is the temperature of the groundwater under the beach and inland”. A minimum temperature within the sands at the back of the beach must be warmer than 20oC for at least half a year.
Returning now to my sample from Fishermans Beach. When he heard it was from Sydney he said it could not be pure beachrock. His Australian trip in 1961 showed that the southernmost locality was at Mackay in Queensland. He measured the winter groundwater temperature here at 200C. This rock was “poorly developed”, but “excellent banded beach rock” occurred north of Townsville where the temperature was measured at 22oC. He also stated that he found no beachrock in New Zealand as the “groundwater is too cold for beach rock to develop. A reported occurrence in Auckland Harbour was a misidentification”.
I was disappointed with his perfunctory dismissal of my sample without further consideration of its local features (Ca CO3 content c.30% plus freshwater seepage). But I was a new graduate student and not in a position to argue. Today both Andy Short and I are convinced it is worthy of designation as “pure” beachrock just as Roger McLean is satisfied that the material near Auckland deserves a similar interpretation. This experience of rejection added to my growing scepticism of those with entrenched views knowing even then there were other models of beachrock origin in the literature.
One of the most impressive studies on beachrock which I read in the early 60s was by Clifford Kaye based on his investigation of shoreline features in Puerto Rico (see USGS Prof. Paper, 317-B, 1959). Detailed mapping alerted Kaye to uncertainties associated with different hypotheses of beachrock formation. He was cautious in reaching conclusions, for instance “The striking correlation between limestone coastal areas and beachrock may conceivably be fortuitous, or else the result of a somewhat indirect correlation”. Unlike what Russell found, Kaye did not observe any dilution effects of sea water at most of his sites. Furthermore Kaye did not reject microbial processes noting that the distribution of beachrock could suggest very specific conditions for responsible organisms. What he did find in agreement with Russell was that most beach rocks were cemented by granular calcite not acicular or needle-like aragonitic carbonate.
It is fascinating that Kaye’s USGS colleagues at Bikini were finding their samples cemented more by aragonite that calcite. This suggested the precipitated cement was derived from seawater while calcite is more common where freshwater is available. Terry Scoffin and Roger McLean studied several beachrock sites on the northern Great Barrier Reef Expedition in 1973 and found the most common cement fabric in the leeward cay beach rocks was that of a thin fringe of acicular aragonite crystals around sand grains (in Phil. Trans. Roy. Soc. London, 1978, 291). They note that carbonate sand grains in the samples show no sign of post-depositional solution so the grains themselves cannot be a source of the cement. At these cay locations the vertical span of modern beachrock relates closely to the tidal range indicating a “direct control by daily soaking and evaporation”.
Questions around processes and age of cementation add to the enigma. Observations in many places show how fast the process can occur. Emery, Tracy and Ladd in their incredible 1954 Bikini report describe WW11 shell fragments as being “now (1951) firmly cemented into the beach rock”. This is one of the remarkable characteristics of beachrock. While in Greece last year, Colin Woodroffe heard of experiments in creating the material with the aim of mitigating foreshore erosion. What is happening at sites where beachrock occurs outside those areas of active formation in tropical locations needs more attention. This includes sites described by Andy Short in southern Australia (e.g. near Coombra, SA1420 in Australian Coastal Systems, 2020, p.966). Beachrock levels as indicators of relative sea level change also present challenges as discussed by Colin Murray-Wallace and Colin Woodroffe (Quaternary Sea-Level Changes, 2014, p.126). All in all, quite intriguing.
Words by Prof Bruce Thom. Please respect the author’s thoughts and reference appropriately: (c) ACS, 2024. For correspondence about this blog post please email firstname.lastname@example.org