Hout Bay Field Trip

The cape Granite is an intrusive igneous rock that has been exposed to the surface of the Earth, formed by solidification of magma beneath the Earth’s surface. The rock contains minerals; feldspar, quartz and biotite, and it is also coarse grained, as evident with igneous intrusive rocks. The feldspar grains are approximately 6 cm, with a white colour.

Incoming waves have an approximate height and period of 0.65 m and 9 s respectively. There is a longshore drift of sand, which is on the west direction, as the prevailing wind’s direction is south east, with the ocean waves also moving in a north westerly direction, thus causing the sand to move in the west direction

The beach sand is fine grained, mainly composed of feldspar and other minerals which were not visible to distinguish, with small grains of sea shells. Detrital grains are formed from rocks or small particles of rocks and biogenic grains are formed from sea shells. The sources of detrital grains are the igneous rocks and of the biogenic grains are the sea shells. Most of these shells will be broken down into smaller grains thus forming part of the beach sand.

Groundwater act also as a source to the Disa river, as the water table has to lie above the river bed allowing for a base flow, where underground water flows into the river. The river bed has ripples on the side of deposition of sediments. The velocity of the river changes or is different as it is flowing on the erosional and depositional banks, forming bed forms which are variable. The sand on the beach is blown away by the prevailing wind, with a direction of south east. The south east direction is the predominant wind direction, since the sand is transported to the north west direction in larger quantities.

Sandstone. It is a highly weathered surface than the granite outcrops at Hout Bay and it also fine grained, resembling characteristics of a sedimentary rock. The chemical reaction taking place is the conversion of silicates to clays, leading to an increase in cracks in the rock as the rock, enabling physical weathering of the rock as the rock is weakened, and temperature changes also causing expansion and contraction of the rock.

The sandstone that is represented on the picture has ripples on it which indicate that when compression of sand was taking place, large masses of water were responsible for such. The mud cracks filled with sand indicate subaerial exposure, as the sedimentary rocks has been finally compacted, cracks within the rocks were filled sand.

The direction of the swell is south east or the swell is coming from the south east, as it approaches the beach face, most of the wave energy is released. When the waves enter the bay they decrease in height and energy.

The Graafwater Formation sedimentary rocks lie above the granite rock. The granite is older than the Graafwater Formation since it was first formed as it crystallised from magma and as years goes by sand was compressed on top of the granite forming the Graafwater Formation.

The trapping of sand in dunes has resulted in an increase in the dunes and the amount of sand trapped in the dunes, thereby increasing the amount of sand that will reach the Sandy Bay and Clifton beaches, and which will result in the neighbouring or surrounding houses being swallowed by the sand. With respect to the direction of the wind which is SE that will result in the sand being blown to the NW direction and that will cause the area to be fully covered with sand, therefore it is not advisable to build permanent structures directly on a beach.

Cite this page