This Earthcache brings you to Imbiah Lookout, where you will find Sentosa Nature Discovery.
<The above is an image of the entrance to Sentosa Nature Discovery.>
Getting There
Admission into Sentosa Nature Discovery is free-of-charge for all visitors. It is open every day from 9 am to 5 pm. You may get to the location via the following means:
- If you're driving, the nearest carpark is located at Imbiah Lookout Car Park.
- By Sentosa Express: Alight at Imbiah Station
- By Public Bus: Board Bus 123 and alight at Village Hotel
- By Intra-Island Bus: Board Sentosa Bus A or C and alight at Imbiah Lookout
- By Cable Car: Board the cable car from HarbourFront Tower 2 or Mount Faber Station and alight at Imbiah Sentosa Cable Car Station
Upon reaching Sentosa Nature Discovery, take the stairs to level 1 where you will find the Geology Gallery, which details the geological history of Sentosa Island, and is the subject of this Earthcache.
<The above is an image of the Geology Gallery at Sentosa Nature Discovery.>
The Geology of Sentosa Island
The Southern Islands (in which Sentosa Island is part of) are formed from part of a large sedimentary rock group deposited during the Mesozoic era (250 to 65 million years ago). There are two distinct parts of the deposit, known as the Jurong Group and the Sentosa Group, based on the time period in which they were formed. The latter forms the bedrock of the Southern Islands, and will be the focus of this Earthcache.
The Sentosa Group, which was deposited in the Upper Triassic Period is further subdivided into the Tanjong Rimau Formation (the cape at the north-western tip of Sentosa) and Fort Siloso Formation, named for the location of the type specimen used to classify the rocks.
There is much that can be said about the formation and types of sedimentary rocks, but we won’t go into exhaustive detail here. For this cache, only a few key points are of relevance:
- The rocks of the Sentosa Group are clastic sedimentary rocks, which means that they are made from the weathered remnants of other rocks that has been eroded, transported and deposited by natural elements.
- The grain size of the sedimentary rock, which is the size and shape of the pieces of original rock/minerals (i.e. clasts) making up the rock, determines the identity of the rock.
- The largest-grained sedimentary rocks, conglomerate, are made out of a mixture of sand and gravel. They are generally larger than 2mm in diameter and visible to the eye. These look like composite rocks cemented together, and some of the original rocks' character, like colour and texture can still be made out.
- As the grain becomes smaller, the rock transitions to sandstone, which are made up sand-sized grains. The grains feel rough to the touch, and have diameters between 62.5 to 2000 micrometers.
- Even smaller grained rocks are the mudstones, which are made from very fine clay particles, which are between 4 to 62.5 micrometers. These look like lumps of unbaked clay, or the surface of unglazed pottery.
- Lastly, we have our tinest claystones, which are made up of even smaller clay sediments of smaller than 4 micrometers in diameter and can't be seen with the naked eye.
- The presence of each of the above types of rocks tell a story of the environment that they were deposited in.
- Larger-grains rocks tend to be deposited in river (or fluvial) environments which has enough flow and energy to transport large pebbles.
- Smaller grain rocks tend to be deposited in lakes (or lacustrine) and meandering river environments where the flow has slowed down sufficiently for the grains to be deposited.
- The smallest grain rocks tend to be found in river deltas (or deltaic) and sea (or marine) environments, where the water has nearly lost all its forward momentum.
- Finally, the colour of the rock paints a picture of the mineral composition of the rock. The mineral composition will not be a focus of this EC, but will serve as an important identifier of the formation the rock belongs to.
With the key characteristics of the rocks laid out, let us turn to the two formations that we will be examining in the Earthcache: the Tanjong Rimau Formation and the Fort Siloso Formation. These two formations are exposed to various extents on Southern Islands, and will form the backdrop to the rest of the geological structures and phenomena you will observe on these islands.
Earthcache Tasks
Task 1: Observe and get to know more about the different types of sedimentary rocks.
Observe the rocks in the “Sentosa Rocks!” display. Describe the colour, grain size and any other visual characteristics of each of the four types of sedimentary rocks. The notes provided above should provide you with some assistance. You will need this information for Task 2.
Task 2: Search for the core samples (cylindrical “tubes” of stones) located near the entrance of the gallery.
Observe the core samples of the two formations: Fort Siloso and Tanjong Rimau and try to identify the sedimentary rocks that make up the formation using the rock samples you have observed and learnt about in Task 1. Justify your answer with the similarities observed in terms of colour, texture and grain size.
Task 3: Learning about the Origins of the Rock Formations
Having identified and learnt more about the composition of the rock formations in Task 2, using your new found information coupled with description notes above on the environment of rock formation, deduce what processes may have produced the sediments that made up the rocks in the two formations. Is it fluvial (from a river) or deltaic/marine (from the sea)? Explain your thought process.
Once you have your answers, either use the Geocaching Message Centre or email to send the answers. Don’t worry about getting all the answers right, but do make a reasonable attempt at the questions as well as a produce proof that you were on-site to make the observations. You may log once you submit your answers and we will be in touch if there are any issues. In the meantime, rock on ;)