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An earthcache highlights geological features of our earth. There is no geocache container to find at this earthcache site!
It would be helpful if you take a measuring tape or ruler with you. Also the subject of this earthcache is often covered at high tide so plan to visit between half tide and low tide.
What is Conglomerate?
Conglomerate is a clastic sedimentary rock. It contains rounded rocks or pebbles (the clasts) that are larger than two millimeters in diameter. The space between these clasts is generally filled with smaller particles and/or a chemical cement that binds the rocks and pebbles together.
Conglomerates consist of rounded clasts and are thus differentiated from breccias which consist of angular clasts.
What is the Composition of Conglomerate?
Conglomerate can have a variety of compositions. As a clastic sedimentary rock it can contain clasts of any rock or other material that is washed downstream or moved by ocean waves or currents. The rounded rock clasts of conglomerate can be mineral particles such as quartz or they can be sedimentary, metamorphic or igneous rock fragments. This conglomerate also has rounded black basalt clasts.
The matrix that binds the large clasts together can be a mixture of sand, mud and chemical cement.
How Does Conglomerate Form?
Conglomerate forms where a sediment of rounded clasts accumulates. It takes a strong water current to transport particles this large, in this case a beach with strong waves. The rounded shape of the clasts reveal that they were tumbled by moving waves.
Conglomerates often begin by being deposited as a sediment consisting mainly of small rocks or other water-transported materials (the 'clasts'). The finer size sand and clay is often deposited later on top of the clasts and then sifts down between them to fill the spaces and form a cement.
Most cementation is the result of new minerals forming in the pore space between sediment particles. The most common cements are quartz, calcite, clay minerals and hematite.
For example, if iron is present in the sediment deposits, hematite cement is formed by the oxidation of iron from its ferrous ion (Fe+2) to its Ferric ion (Fe+3). Precipitation of Fe+3 results in hematite forming (Fe2O3) which is insoluble in oxygen-rich water and cements the sediment particles together.
Other minerals like pyrite, gypsum and barite can also form cements under special geologic conditions.
Only small amounts of hematite coating a mineral grain or rock surface is sufficient to give a red coloured stain.
Caretta Beach
Most of the rocks along the coastline here are black basalt rocks of volcanic origin, rounded by the action of waves. The conglomerate on this beach stands out as being very different from these surrounding rocks. The sand on the upper beach has a reddish/tan hue, due to these sands originally being part of the red ferrosol soils around the Hummock (a nearby remnant volcanic plug). These soils were derived from volcanic basalt rocks, the red colour being mainly due to ferrous minerals in the soil.
The earthcache
To claim this earthcache you must email me answers to the following questions:
- About 20 metres to the North of the main conglomerate slab there are a few large outcrops of conglomerate that would at one time have been attached to the main conglomerate slab. What is the thickness of the northern side of the northernmost of these outcrops?
- Note the colour of the conglomerate, how does it compare to the sand on the nearby beach. Why do you think the conglomerate is this colour?
- Go to S24 48.153 E152 27.286. There is more conglomerate here but it looks different in composition to the conglomerate you have just visited. Look at it carefully and tell me what is different about the clasts embedded in this conglomerate rock.
- Optional: Post a photo of yourself or your GPS with the main slab of conglomerate in the background. Please do not post close-up photos of the northern outcrop or the conglomerate at the coordinates in 3 above.