The fault zone.
The vallies occupied by the Kowloon Reservoir (to the north-east) and Kowloon Byewash Reservoir (to the south-west) are carved in the landscape by erosion along the Lai Chi Kok-Tolo Channel Fault. As the name indicates this is a fault zone that runs from Lai Chi Kok to the Tolo Channel. It continues south-west from Lai Chi Kok under the sea, and becomes visible again as the pass between Mui Wo and Pui O, the low area between Chi Ma Wan peninsula and the rest of Lantau Island.
Here it is responsible for the almost straight and very deep vallies that now form the reservoirs, see the map below. The major faults are indicated by the thick black lines. Further north-west along this fault line lies Sha Tin (which is mostly relaimed land), and it continues along the Tolo Channel. Fault zones typically weaken rock, causing such a zone to erode faster than the surrounding areas, resulting in the deep, sharp, long vallies and narrow sea channel we see today.
Two reservoirs.
These two reservoirs were constructed in natural valleys to supply the growing population of Kowloon with a steady drinking water supply. The two reservoirs form an almost straight line, this is particularly well visible for the Kowloon Byewash Reservoir. The reservoirs occupy a deep valley with steep slopes, caused by erosion along the fault line.
The shape of the reservoirs and indeed the fault zone can be seen very well on satellite imagery. Zoom out or check the geological map to see the alignment with Sha Tin and the Tolo Channel. The straight line of the Shing Mun river channel in Sha Tin is artificial, before the development of Sha Tin New Town in the 1970s this area was a bay. Of course the overall straight shape of the Tai Wai and Sha Tin area is the direct result of erosion along this fault.
Two plutons.
A pluton is a body of intrusive igneous rock (called a plutonic rock) that crystallised from magma slowly cooling below the surface of the Earth. The reservoirs lie on the border between two plutons, which have a slightly different rock composition. Being intrusive rocks they are formed well under the surface, however may become exposed due to deep weathering of the material above them, like what happened in this area.
North of the reservoirs is the Needle Hill Granite, and forms an elliptical biotite monzogranite pluton on the northwestern side of the Shing Mun Valley and in northwestern Kowloon. It mainly consists of porphyritic fine-grained granite and equigranular medium-grained granite. This granite was formed about 146.4 million years ago.
South of the reservoirs is the slightly younger (formed about 146.2 million years ago) Sha Tin Granite which forms an irregular elliptical-shaped biotite monzogranite pluton centred on the Sha Tin district. The rocks consist mainly of equigranular coarse- and fine- to medium-grained biotite granite.
These formations are outlined on the map below.
Original size.
At the cache site.
The coordinates take you to roughly the middle of the main dam separating the Kowloon Reservoir from the Kowloon Byewash Reservoir. This is the best viewpoint over the reservoirs.
Looking south-west over the Byewash reservoir you can clearly see the very distinct straight shape of the valley. The reservoir ends at the dam, behind that you can look in between the mountains across the port of Kwai Chung and the Stonecutter's Bridge. On a clear day you may be able to see the Chi Ma Wan peninsula.
Looking in the opposite direction over the Kowloon Reservoir you can again see the valley continue. Here you are looking over the longest arm of the reservoir, and towards the lowest point in between the mountains. As the mountains are a little higher in that direction, Sha Tin is not visible.
Logging tasks.
- At the reservoirs the border between the plutons and the fault zone coincide. What effect, if any, you think this may this have on the weathering that resulted in the pronounced straight and narrow shape of especially the Kowloon Byewash Reservoir?
- The slopes next to the reservoirs are covered in thick vegetation. Keeping this vegetation healthy and strong is of vital importance for keeping the reservoirs and with that our drinking water clean - construction of the Tai Lam reservoir actually went hand in hand with an extensive reforestation project on the adjacent slopes. Why is that vegetation cover so important?
Getting there.
Take KMB bus 72, 81 or 86B, get off at stop "Shek Lei Pui Reservoir", and walk for about 500m downhill along Golden Hill Road to the cache location.
Final notes.
The dam is narrow, but still open for vehicles, so be careful. The place is wheelchair accessible, there are a few places near the dam where a car can be parked. The area is also full of monkeys, please don't feed them (and keep all your food hidden from them). They're aggressive enough already and feeding makes matters worse. And feeding wild animals is illegal, by the way. Enjoy the area, and enjoy the monkeys - they're generally harmless and fun to watch.
Send your answers to the above questions to me by personal message, do not post answers in the log. Logs with answers will be removed without prior notice.
The most exciting way to learn about the Earth and its processes is to get into the outdoors and experience it first-hand. Visiting an Earthcache is a great outdoor activity the whole family can enjoy. An Earthcache is a special place that people can visit to learn about a unique geoscience feature or aspect of our Earth. Earthcaches include a set of educational notes and the details about where to find the location (latitude and longitude). Visitors to Earthcaches can see how our planet has been shaped by geological processes, how we manage the resources and how scientists gather evidence to learn about the Earth.
Sources.
HONG KONG LANDSCAPES Shaping the Barren Rock, B. Owen and R. Shaw.
Geological map of Hong Kong.