Introduction

Knowledge of the geological history and structure of the region can be useful when planning dives in this region, as an understanding of the subject can allow a better prediction of conditions and environment to be expected during the dive.

Recreational divers

Recreational diving

Recreational diving or sport diving is a type of diving that uses SCUBA equipment for the purpose of leisure and enjoyment. In some diving circles, the term "recreational diving" is used in contradistinction to "technical diving", a more demanding aspect of the sport which requires greater levels...

 will usually dive in the shallow to intermediate marine environment. Technical and commercial divers may venture into the deep water environment.

Shallow water is defined as between the surf-zone and the coast, whereas intermediate water is defined as between the surf zone and wave base (where the waves just interact with the bottom and no more, usually about 80m water depth with 10 second swells).

Deeper than wave base is defined as deep water: i.e. too deep for waves to interact with the seafloor.

The seafloor beneath intermediate water is termed the shoreface and is the zone where the seafloor slows down the swells by friction, so that the surf ends up being lower than it otherwise would be and hence there is less coastal erosion than there otherwise would be.

Over the last 2 million years of the Quaternary

Quaternary

The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...

 geological period, cool glacial period

Glacial period

A glacial period is an interval of time within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate within an ice age...

s (hypothermals) about 100 000 years long, have been the norm. Canada and northern Eurasia were covered by continental ice sheets kilometres thick, and the global effect was a lowering of sea level by some 130m because the sea was the source of the frozen water of these huge ice sheets. This means that False Bay

False Bay

False Bay is a body of water defined by Cape Hangklip and the Cape Peninsula in the extreme South-West of South Africa.- Description and location :...

 and Table Bay

Table Bay

Table Bay is a natural bay on the Atlantic Ocean overlooked by Cape Town and is at the northern end of the Cape Peninsula, which stretches south to the Cape of Good Hope. It was named because it is dominated by the flat-topped Table Mountain.Bartolomeu Dias was the first European to explore this...

 were dry and covered by dunes for 90% of the last 2 million years. Warm interglacial

Interglacial

An Interglacial period is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age...

 periods (hyperthermals), have lasted only about 10 000 years and we are part way through the latest one which started about 6000 years ago. The coastline of False Bay, therefore, was usually across the mouth of the present False Bay. Robben Island was a hill on a coastal plain with the coast west of the island for most of the past 2 million years.

Malmesbury group deposition

The late-Precambrian

Precambrian

The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...

 age Malmesbury group is the oldest rock formation in the area, consisting of alternating layers of dark grey fine-grained greywacke

Greywacke

Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...

, sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

 and slate

Slate

Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...

, seen along the rocky Sea Point

Sea Point

Sea Point is one of Cape Town's most affluent and densely populated suburbs, situated between Signal Hill and the Atlantic Ocean, a few kilometres to the west of Cape Town's Central Business District . Moving from Sea Point to the CBD, one passes through first the small suburb of Three Anchor Bay,...

 and Bloubergstrand shorelines, and from the Strand to Gordon's Bay

Gordon's Bay

Gordon's Bay is a harbour town in the Western Cape province of South Africa, close to Strand. It is situated on the northeastern corner of False Bay about 50 km from Cape Town to the south of the N2 national road and is named after Robert Jacob Gordon , the Dutch explorer of Scottish descent...

. These sediments were originally deposited on an ancient continental slope by submarine slumping and turbidity current

Turbidity current

A turbidity current is a current of rapidly moving, sediment-laden water moving down a slope through water, or another fluid. The current moves because it has a higher density and turbidity than the fluid through which it flows...

s. The sequence was subsequently metamorphosed by heat and pressure and folded

Fold (geology)

The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...

 tightly in a NW direction during the Saldanian orogeny

Orogeny

Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...

 so that the rock layers are now almost vertical. These rocks were, in most places, scoured by wave action during past periods of higher sea level.

Appearance of the exposed Malmesbury group rocks

Most of the exposed shoreline Malmesbury rocks are steeply dipped, and weathered to form sharp edged ridges where more resistant layers stand out among the softer strata. The rocks are generally dark in colour where fresh rock has been exposed by erosion, and may be finely laminated.

Coastal areas where Malmesbury group rocks are exposed

• Robben Island
• Green Point to Sea Point
• Gordon's Bay
• Blousteen (between Koeelbay and Rooi-els)

Peninsula granite intrusion

The Peninsula Granite

Granite

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...

 is a huge batholith

Batholith

A batholith is a large emplacement of igneous intrusive rock that forms from cooled magma deep in the Earth's crust...

 that was intruded into the Malmesbury Group about 630 million years ago as molten rock and crystallized deep in the earth, but has since then been exposed by prolonged erosion. The characteristic spheroidal shapes of granite boulders are a result of preferential weathering along intersecting fractures and are well displayed around Llandudno

Llandudno

Llandudno is a seaside resort and town in Conwy County Borough, Wales. In the 2001 UK census it had a population of 20,090 including that of Penrhyn Bay and Penrhynside, which are within the Llandudno Community...

 and Simonstown. Close up, the granite is a coarse-grained rock consisting of large (2–5 cm) white or pink feldspar

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....

 crystals, glassy brown quartz

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 and flakes of black mica

Mica

The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...

, and containing inclusions of dark Malmesbury hornfels

Hornfels

Hornfels is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat of intrusive igneous masses and have been rendered...

.

The climate of this region was warmer and wetter in the Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

. This led to severe chemical weathering of the granite to saprolite

Saprolite

Saprolite is a chemically weathered rock. Saprolites form in the lower zones of soil profiles and represent deep weathering of the bedrock surface. In most outcrops its color comes from ferric compounds...

 rich in kaolin clays, decomposed from the large visible crystals of potassium feldspar that are so conspicuous in the granite.

The contact zone where the Malmesbury Group was intruded by molten granite can be seen at Sea Point and was made famous by Charles Darwin

Charles Darwin

Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

 during his voyage of scientific discovery on H.M.S. Beagle in 1844. Here, slivers of dark coloured Malmesbury rocks, altered by intense heat are intermingled and folded with the pale coloured intrusive granite to form a complex mixed rock. Large feldspar crystals occur in both the granite and dark hornfels layers

Though initially intruded at great depth, prolonged erosion eventually exposed the granite at the surface and it and what remains of the similarly eroded Malmesbury group now form a basement upon which younger sedimentary rocks of the Table Mountain Group were deposited.

Other large granite plutons of similar age are found in the Western Cape

Western Cape

The Western Cape is a province in the south west of South Africa. The capital is Cape Town. Prior to 1994, the region that now forms the Western Cape was part of the much larger Cape Province...

, but none of the others extend to the coastline in this area. The Stellenbosch pluton extends under the Helderberg and Hottentots Holland mountains. Cape Blue Rock, a dense Hornfels

Hornfels

Hornfels is the group designation for a series of contact metamorphic rocks that have been baked and indurated by the heat of intrusive igneous masses and have been rendered...

, was formed from the Malmesbury series rock by the Stellenbosch pluton, and was quarried as a building aggregate near Sir Lowry’s Pass

Sir Lowry's Pass

Sir Lowry's Pass is a mountain pass on the N2 national road in the Western Cape province of South Africa. It crosses the Hottentots-Holland mountain range between Somerset West and Grabouw on the main national road between Cape Town and the Garden Route...

. The Blue Rock Quarry has been closed and is now flooded and used for water sports including diving and water skiing.

Appearance of the exposed Peninsula Granites

Almost all the exposed granite has been extensively weathered and is in the form of rounded corestones. The colour is generally pale to medium grey, and the surface is typically fairly rough, with clearly visible crystals, and no layered structure. As an intrusive rock, dip and strike do not apply, but the massive rock is cracked on jointing planes, which tend to be characteristic of the location, and weathering has accentuated these joints. The general direction and spacing of joints in some areas is fairly consistent over quite large areas, and the underwater landscape is often a continuation of the general trends above the surface, which can be useful for navigational purposes.

Coastal and offshore areas where Peninsula Granite rocks are exposed

• Sea Point to Chapman's Peak
• Simon's Town harbour to Partridge Point
• Roman Rock and adjacent reefs
• Cape Point
• Bellows Rock (off Cape Point)
• Seal Island (False Bay)
• Whittle Rock (offshore reef in False Bay)

Table Mountain Group deposition

Table Mountain Group sandstones were deposited on the eroded surface of granite and Malmesbury series basement, in the stream channels and tidal flats of a coastal plain and delta environment that extended across the region about 450 million years ago. The sand, silt and mud deposits were lithified by pressure and then folded during the Cape Orogeny to form the Cape Fold Belt

Cape Fold Belt

The Cape Fold Belt is the folded sedimentary sequence of rocks in the southwestern corner of South Africa. It is related to the Ventana Mountains near Bahía Blanca in Argentina...

, which extends along the western and southern coasts.

The basal Graafwater Formation (300-450m thick) consists of interlayered pale brown sandstone, laminated pink siltstone

Siltstone

Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...

 and dark maroon coloured shale

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

. Closer examination shows deposition cycles from current-bedded channel sandstones to increasing proportions of fine-grained maroon shales at the top, deposited in flood plains and lagoons.

The Peninsula Formation (800-1500m thick) consisting of hard, light grey medium to coarse grained pebbly quartz sandstone, dominates the steep mountain cliffs. Current bedding and pebble layers suggest that it was originally deposited as migrating sand bars in broad river channels.

The Pakhuis Formation tillite (a lithified glacial outwash gravel) occurs on the highest points of Table Mountain, such as Maclear’s Beacon and parts of the Hottentots Holland range. It contains clusters of angular boulders and pebbles and was deposited at a time when the Gondwana

Gondwana

In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...

 continent, of which Africa was a part, was situated close to the south pole.

Graafwater, Peninsula and Pakhuis formations are from the Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 period.

Cedarberg, Goudini and Skurweberg Formations from the Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 period, and Rietvlei Formation from the Devonian

Devonian

The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...

 period complete the Table Mountain group, and are found in the Hottentots Holland mountains to the East of False Bay. These strata are all well above the present sea level.

The Cedarberg Formation includes dark grey siltstone and silty sandstones, and is darker than the overlying Goudini Formation which is light brown to light grey quartzitic sandstone, with interbedded

Interbedding

In geology, Interbedding occurs when beds of a particular lithology lie between or alternate with beds of a different lithology. For example, sedimentary rocks may be interbedded if there were sea level variations in their sedimentary depositional environment....

 reddish brown siltstone and shale. The Goudini formation is more feldspathic and finer grained than the Skurweberg Formation, which is characterised by fairly thick bedded medium to coarse grained, light grey, slightly feldspathic quartzose sandstone.

The Rietvlei Formation consists of alternating horizons of light grey quartzose and feldspathic sandstone, siltstone and shale.

Cape orogeny

The formation of the Cape Fold Belt is the result of a collision of tectonic plates

Tectonic Plates

Tectonic Plates is a 1992 independent Canadian film directed by Peter Mettler. Mettler also wrote the screenplay based on the play by Robert Lepage. The film stars Marie Gignac, Céline Bonnier and Robert Lepage.-Plot summary:...

 that ended over 200 million years ago The accumulated strata of the Cape Supergroup and the older granites and Malmesbury group were raised and deformed by the pressure of the South American, Antarctic and African continental plates slowly moving together. The resulting fold mountains have been eroded to their present state over the ensuing period, and what exists today are the remnants of a much larger and higher mountain chain.
Faults cut across and displace the rock layers. These more easily eroded zones are marked by ravines; cross-cutting faults separate multiple peaks of the Twelve Apostles. Some fault zones of crushed rock (breccia) are re-cemented by dark brown coloured iron and manganese oxide minerals.
The present landscape is due to prolonged erosion having carved out deep valleys, removing parts of the once continuous Table Mountain Group sandstone cover from the Cape Flats and leaving high residual mountain ridges.

Appearance of the exposed Table Mountain sandstones

Strike and dip vary considerably: on the Cape Peninsula dip is generally near to horizontal, so there are ledges rather than ridges, but on the east side of False Bay the dip is steep, often more than 45°, and where the strike is parallel to the shoreline (near Gordon's Bay and Rooi-els), this results in underwater ridges and gulleys also parallel to the shoreline, and usually interrupted by transverse gullies which are extensions of gullies extending above the water. In other places the ridges are at a greater angle to the shoreline.

The offshore reef at Steenbras Deep is relatively flat topped, and the strike and dip are not recorded.

The extensive shoal at Rocky Bank is also relatively flat topped, with steps, and here, too, the dip is fairly flat.

Coastal and offshore areas where Table Mountain Group rocks are exposed

• Noordhoek to Cape Point
• Smitswinkel Bay to Cape Point
• Simon's Town to Muizenberg
• Gordon's Bay to Koeelbay
• Rooi-els to Cape Kangklip
• Rocky Bank (offshore reef in mouth of False Bay)
• Steenbras reef (not confirmed: Sedimentary rock, but possibly Malmesbury group)
• East shoal (not confirmed: Sedimentary rock with strike roughly N-S magnetic, and dip to east roughly 30°. The rocks of East shoal are visually similar to Rocky Bank)

Tertiary to Recent events

Almost 50% of the Cape Peninsula and Cape Flats area is covered by weakly cemented marine sands. Sea-levels fluctuated between -120 to +200m from present mean sea level during the Pliocene and subsequent Pleistocene ice-age between 2 million and 15000 years ago as a result of fluctuating global temperature and variable amounts of water accumulated in polar ice caps.
At times the sea covered the Cape Flats and Noordhoek valley and the Cape Peninsula was then a group of islands. Beach sands with shell fragments and estuarine muds were deposited and later overlain by calcrete-cemented dune sands as the sea retreated. "Dune rock" that was deposited during a Pleistocene interglacial period about 120 000 years ago is now being eroded in the sea-cliffs near Swartklip.

During glacial periods the sea level dropped to expose the bottom of False Bay to weathering and erosion. The last major regression, about 20,000 years ago, lowered the sea level to the present 130m isobath, which is south of Cape Point and Cape Hangklip, leaving the entire bottom of False Bay exposed. During this period an extensive system of dunes was formed on the sandy floor of False Bay. The southward extensions of the now-buried Tertiary drainage must run east of Seal island, York Shoal and East Shoal in the north-central part of False Bay, and then down its central bathymetric deep axis towards the outlet between the submerged features of Rocky Bank and Hangklip Ridge.

Bottom sediments

The bottom off the west coast of the cape Peninsula between and beyond the rocky reef areas is largely fairly fine white quartzitic sand with some areas of coarser shelly sand.

The bottom sediments of False Bay are more varied. On the west side of the bay there is a general tendency towards fine to medium quartzitic sand and coarser calcareous material, mostly mollusc shell fragments, with patches of a maerl of branching coralline algae fragments. There are also areas of very fine sand, almost mud, in the more sheltered Simon's Bay.

The east side of False Bay also has extensive areas of fine quartzitic sand bottom, and there are also occasional deposits of a very fine low density silty material which is easily disturbed and returned to suspension. This silt generally deposits over the sand and depressions in the reef during relatively long periods of calm seas. There are small areas of shelly gravel and mud near Gordon's Bay between the rocky shoreline reef and the sand bottom.

Marine terraces

Gravel beds underlying raised-beach terraces

Marine terrace

A marine terrace, coastal terrace, raised beach or perched coastline is a relatively flat, horizontal or gently inclined surface of marine origin, mostly an old abrasion platform which has been lifted out of the sphere of wave activity . Thus it lies above or under the current sea level, depending...

 occur intermittently along most of the coastline. Boulder beds are found almost continuously from Kommetjie southwards to Cape Point They rest on a terrace, 7 to 10m above sea level, which cuts into Table Mountain Sandstone. There are indications of an older raised pebble beach at 17 to 20m here as well.

Along the False Bay coast evidence of higher stands of the sea is presented by wave cut platforms, benches, boulder-beach ridges, sea cliffs, caves and undercut ledges Wave cut caves, indicating former sea level stands between +4 and +8m are found between Cape Point and Muizenberg at Rooikrans, Batsata cove, Blaasbalkgrot and Hell’s Gate, among others. Even more spectacular wave-cut caves occur on the Eastern side of False Bay from Gordon’s Bay to Pringle Bay, eg at Die Kamer, Sandgat, Heuningkloof, Koeëlbaai, Blousteen, Rooiels and Drostersgat.

Wave cut platforms at 18 to 20m and at 30m are well developed below Clarence Drive from Steenbras River Mouth to Koeëlbaai. On them rest rounded sandstone boulders, gravel and sand, up to a metre thick, often cemented by manganese and iron. In places scree partially overlies these deposits. In the same vicinity raised boulder beaches, similar to those on the peninsula, occur at 3 to 5m above sea level.

Agglomerate and basaltic rocks

On the False Bay coast just below the old lighthouse at Cape Point, a basic agglomerate

Agglomerate

Agglomerates are coarse accumulations of large blocks of volcanic material that contain at least 75% bombs...

 plug intrudes course grained porphyritic granite. This outcrop is unconformably overlain by sediments of the Table Mountain group. It consists of a heterogeneous mass of dark green to grey basic rock with red-brown specks that enclose randomly oriented clasts of variable size up to 300mm. The clasts consist of rounded and angular quartzose sandstone, siltstone, hornfels, porphyritic

Porphyritic

Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group...

 granite, basic rocks and fine-grained reddish granite. Basic dykes of similar lithology

Lithology

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples or with low magnification microscopy, such as colour, texture, grain size, or composition. It may be either a detailed description of these characteristics or be a summary of...

 to the matrix of the plug, cut both agglomerate and adjacent granite

Dolerite dykes

There are a number of dolerite dykes

Dike (geology)

A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...

 in the region, some of which are visible at the coast. These dykes are intrusive to the Cape Granite Suite and pre-Cape sedimentary rocks, and in some places also the Graafwater and Peninsula formations. They vary in width from a fraction of a metre to as much as 22m at Logies Bay, Llandudno.

Most of the dolerites are fine to medium grained, dark grey rocks with augite

Augite

Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...

 and plagioclase

Plagioclase

Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...

 as major constituents. Quartz enriched variations of dolerite are found at Miller’s Point

Miller's Point, Western Cape

Miller's Point is a headland and stretch of protected coastline in South Africa. It is located about south of Simon's Town on the road to Cape Point.-History:...

, Llandudno and Chapman’s Peak.

External links

• Geology of the Cape Peninsula: Department of Geological Sciences, University of Cape Town