Arniston Beach Rocks!
This EarthCache is located on the coast just south of Arniston en route to the famous and spectacular Waenhuiskrans Cave which is a short distance south on the next rocky headland.
These beaches really do ‘rock’ as they have numerous fascinating features revealing various aspects of beachrock formation and subsequent erosion.
The best time to see these is at or near low tide as those in the intertidal zone may be more or less hidden at higher sea levels. So, do check on one of the many online tide charts, to ensure you maximise the potential benefit of your visit!
To reach the cache locations:
a) Shortest route: park roadside at S 34 40.396 E 20 13.886 on Harbour St heading south along the coast from the village. Then take the path heading down from there to the cache locations.
b) On foot: from the harbour area in front of the hotel follow the paths and roads along the sea front as far as the end of Krantz St @ S 34 40.160 E 20 14.064. From here pick up the small trail which skirts along the top of the rock slope/front wall of the house then around the corner, making your way across and then down to the beach and along this to the approximate cache locations.
Location 1 (edge of intertidal platform): S 34 40.412 E 20 13.962
Location 2 (top of rocky headland): S 34 40.405 E 20 13.993
Location 3 (landward edge of rock outcrop on adjacent beach): S 34 40.363 E 20 13.951
To Claim the EarthCache
Please make the necessary observations on site and answer the following questions, submitting answers to the CO by GC message from my Profile page or email (forshaw.chris@gmail.com):
At Location 1
- You will see many potholes and also a few deep round-bottomed steep-sided channels containing grinders. Explain briefly what the channels are and how they were formed.
- Looking across the intertidal platform from the landward edge towards the sea, what observations can you make about the roughness of its surface?
- You will note that the remaining rounded ‘hills’ in the highly weathered area closest to the landward side all have multiple green-tinged – perfectly horizontal ‘countour’ lines on their sides. What do you think causes these? (Clue: the surrounding shallow depressions are often mini rockpools at low tide)
- From GZ look at the nearby rocky aeolianite headland. Estimate its maximum height above sea level, and estimate the hight and width of the large ‘cave’ in its side.
At Location 2:
- On the seaward side of this headland, the rock has a very distinctive appearance, quite different from elsewhere. How would you describe it – and what are these erosion features called.
At Location 3: Note: on the last visit (Feb 2020) this feature (sea-level notch) had become buried under sand - so don't worry if you can't spot it!
- At this spot you should see a raised area of rock - a low headland - covered on top by a thin layer of green vegetation. You will note the very distinctive shape of its landward side and the fascinating appearance of its almost white surface. Please describe the feature, identify what it is, and suggest the type of rock.
Optional: take and post a photo of yourself admiring some fascinating beachrocks!
The rocks which can be seen from GZ are part of the Waenhuiskrans Formation which is one of the formations constituting the Cenozoic Bredasdorp sediments which occur along the south coast of Western Cape Province.
Cenozoic = the 3rd major era of Earth's history, starting about 66 million years ago and extending to the present. It is when continents assumed their current shapes and positions and flora and fauna evolved toward those of the present.
Formation = a body of rock with consistent physical characteristics (lithology) distinguishing it from adjacent bodies of rock.
Types of rocks present at GZ are:
a) aeolianites which form the rough rocky headlands, cliffs, overhangs and caves. Aeolianite is rock formed by lithification of accumulated carbonate sediment first thown up on the beach by waves, then wind-blown and deposited into coastal dunes. The most common form is coastal limestone. (Aeolian = wind-generated)
b) beachrocks which are rocks of the shallow intertidal zone which form a gently sloping rocky platform with numerous interesting features, particularly those caused by various types of erosion.
Lithification process: both types of rock are formed from sediment which becomes lithified (made into rock) by the precipitation of calcium carbonate (calcite) from the coastal seawater, where it is already at maximum concentration.
Note: in lithification of beachrocks, seawater is the primary source of cement
In the case of the aeolianites, this occurs when the wind-deposited dune sediment is subjected to sea-spray which on exposure to the air/wind and heat, evaporates and concentrates causing the calcite to precipitate as a cement which binds the sediment particles together eventually forming rock.
In the case of the beachrock, lithification occurs in the zone and time between high tide and low tide when beach sediment previously suspended or underwater becomes deposited/exposed on the shore as the tide recedes. Again, subsequent exposure to the air/sun causes calcite to be precipitated from the seawater, binding the sediment particles together to form a hard rocky layer.
Age: most beachrocks have been dated between 1,000-5,000 years old, but data collection issues suggest that the abundance of younger rocks is underestimated. In geological terms, these are very recently formed rocks which are still being formed.
Dimensions and Key Local Features: the The Waenhuiskrans Formation is a narrow 200m-3 km wide discontinuous vegetated, semi-consolidated dune-sand occurring along the coastline between Hermanus and Plettenberg Bay. The type area of the Waenhuiskrans Formation is here at Arniston.
The formation consists of calcarenite* and calcareous sandstone with a carbonate content ranging from 23-92% (depending on the amount of shell material present) - at Arniston it is 86-92%.
The thickness of the Waenhuiskrans Formation varies significantly, with a maximum of 200m and average of 30m. At Arniston it is 15m thick, consolidated, and highly calcareous. Strong wave action has cut well-developed platforms (extensive flat rocks) in the formation and formed large recesses in the rock (like the one on the headland near GZ), and the Waenhuiskrans cave in the up to 25m high sea cliffs, which also has a distinct visor (rock overhang) at the sea entrance to the cave.
*Calcarenite is consolidated carbonate sand formed by the percolation of water through a mixture of shell fragments and sand. It consists mainly of sand-grade (2mm diam) calcitic or aragonitic (crystal forms of calcium carbonate) particles. The term calcarenite not only indicates mineral composition and texture but also carries a genetic connotation.
The Sediment forming beachrock can contain a wide variety of particles, including human artefacts and litter, ranging from ancient pottery fragments to drink cans. Here the main component is mainly of shallow marine biogenic origin (originating from living organisms) - namely calcareous grains of comminuted (reduced to tiny particles) marine shells. These are ubiquitous in marine environments - in this area the most common are mollusc fragments - black mussel shells. The well-sorted sand component consists mostly of medium-grained, well- to very well-rounded quartz grains.
Beachrock Erosion: having been formed, as with any rock subsequently exposed to the elements – wind, rain, heat and particularly in this case wave action – is subject to erosion, which eventually over a prolonged period (depending on how erosion-resistant the rock is) reduces the rock back to grains which accumulate as sediment, starting the whole cycle of sedimentary rock formation again.
Processes: the chemical dissolution of the carbonate cement holding the rock together is caused by pH fluctuation of sea-water trapped on the rock. These fluctuations are temperature dependent as the gas is less soluble in warmer water.
pH can also be altered by marine algae. During daylight they remove CO2 from the sea-water for photosynthesis causing an increase in pH and at night they release CO2 into the water through respiration causing a reduction in pH.
The rock is also eroded by the activities of a large variety of marine organisms living on or in the rock surface (see graphic in Gallery)
Erosion Features: coastal erosion produces characteristic structures and shapes in the intra-tidal and supra-tidal zones (on and above the shore). Of these, the following six features are particularly notable in the area around and adjacent to GZ with a) – d) around Location 1 and e) and f) at nearby Locations 2 and 3:
a) Primary Potholes: (near-)circular depressions of highly variable dimensions physically ground into the rock. These vary from 5cm-2m deep and 10cm-3m across and typically have a diameter to depth ratio <1:4 (ie. for a diameter of 10cm, depth is likely to be <40cm).
They are usually filled to some degree by sandy sediment or larger, well- rounded, rock fragments (grinders) but their walls are almost always colonised by marine algae. Grinders are pebbles and cobbles of resistant rock which act as abrasive tools. Powered by wave energy, they physically grind and abrade the base of the pothole deeper into the substrate (beachrock). However, grinders are not a prerequisite for pothole formation as sandy sediment carried by water is also an effective abrasive.
b) Secondary Potholes: these are potholes formed in the base of existing large(r) primary potholes which are formed by abrasion processes exploiting weaknesses in the rock in which the primary pothole occurs.
c) Elongated Potholes: these are recognisable as narrow deep channels with rounded bases running at 
right-angles (90O) to the beach, often containing grinders. They often originate as primary potholes which become lengthened by the erosive process becoming more dominant in one direction - most commonly due to the 180O reversal of tidal action. They may also be due to tidal action eroding joints or depressions in the rock surface. Micro-organisms trapped in mini rock pools tides also chemically weaken the rock encouraging subsequent further abrasion by the grinders being rolled and tumbled up and down the channels.
d) Intertidal Platforms: near Arniston these are near-flat exposures of carbonate-cemented sandstone found at the mean low water mark. They are terminated on one side by the sea and on the other side by either a sea level notch or by unconsolidated beach sand. They form due to the landward retreat of sea level notches under attack by a combination of chemical, mechanical and biological erosion. They may also be formed by exposure of buried beachrock which underwent diagenesis during a previous sea level still-stand (period when the level remained constant).
e) Sea-level Notches: are concave profiles on the landward margin of intertidal platforms that are eroding back into a headland. At Arniston these are composed of carbonate-cemented sediments. The base of the notch shows the mean low water mark and is thus an important sea level indicator. A combination of chemical, mechanical and bioerosion are operative within the intertidal range of the sea level notch. Above the mean high-water mark, bioerosion by chitons, browsing molluscs and boring molluscs feeding on endolithic algae can account for rates of denudation of greater than 1mm/yr. As a result of bioerosion, the vertical extent of the sea level notch can be more than double the tidal range.
f) Spitzkarren: are upward-pointing pyramid or projectile-shaped bodies of rock separated by clefts. In 
Arniston’s coastal outcrops they are restricted to the supratidal zone in the carbonate sediment headlands where the rock is not subjected to immersion or prolonged streams of water but repeatedly wetted by wave spray, rain splash or mist. This gives their surfaces a very jagged, pitted and irregular appearance.
Sources used:
Beachrock, Wikipedia (https://en.wikipedia.org/wiki/Beachrock)
Beachrock occurrence, characteristics, formation mechanisms and impacts, MI Vousdoukas AF Velegrakis, TA Plomaritis (Science Direct 2007)
Erosional Features of Coastal Beachrock and Aeolianite Outcrops in Natal and Zululand, South Africa, WR Miller and TR Mason (Journal of Coastal Research 1994)
The Stratigraphy and Sedimentology of the Bredasdorp Group, Southern Cape Province, Jean Arnaud Malan (MSc Thesis, University of Cape Town 1990)
Mineralogy of Carbonates: Beachrock (https://www.geological-digressions.com/mineralogy-of-carbonates-beachrock/)
Acknowledgements: Thanks are due to Richard Fuggle (Richard.F), retired UCT Professor of Environmental Studies, for kind and careful review of the draft cache description.