Ping Chau, often known as Tung Ping
Chau, sits in Mirs Bay on the Northeastern side of Hong Kong.
Looking across the Dameisha and Xiaomeisha beaches on the Mainland,
this crescent-shaped outlying island at the most eastern end of
Hong Kong's waters is renowned for its spectacular rock formations
and geological features. It has an area of 1.1 square kilometers.
Hok Ngam Teng (48m) at the southern tip of the island is the
highest point while the central region is a 15-25m tall gentle
terrace. As the variation in height is small, the island is
basically devoid of hill, stream and valley. It only gives people
an impression of a generally flat topography. It is named the best
of Hong Kong's four famous natural sights. Tung Ping Chau is an
island of sedimentary rocks. The name Ping Chau (Flat Island) comes
form the unique formations of such flat sedimentary rock stacks,
with unique landscape and other features of coastal erosion.
Major points of interest are: Kang Lau
Shek, wave-cut platforms, Lan Kwo Shui, Hoi Lo Tung, Lung Lok Shui,
Lung Lun Tsui and Cham Keng Chau. There are ample opportunities of
watching birds, butterflies and dragonflies; studying coastal
plants; and exploring old villages. The ocean-quality water that
surrounds the island provides a good environment for soft corals,
hard corals, alga, inter-tidal fauna and flora. The island is thus
ideal for walking, nature studies and diving.
- Geological History Sedimentary Rocks and Dip Angles
Tung Ping Chau Island is made up of unique bedding called the Ping
Chau Formation. Silt from eroded rocks were washed downed by rivers
and streams and settled on the seabed. Under great pressure, lime
in the sediment cemented to form hard rock bed that was later
brought to surface by crustal movements.
The Ping Chau Formation is primarily made up of dolomite and
calcium shales, marls, calcitic and silicon siltstones. The stratum
is approximately 200m thick. As these rocks often contain minerals
such as zeolites and acmites, it indicates that they were deposited
in a high calcium salt lake quite far from land, or in a lagoon of
high salinity. As pyrite granules and pyrite cakes are often found
in the rocks, it also indicates the sedimentation environment was
quire anaerobic and high in salinity.
Because of crustal movements, the rocks of Ping Chau Formation
ware raised above the sea and inclining gently from west to east at
a dip angle of 10°-19°. The north and south tips of the island also
slant towards the inlet of the Ping Chau Hoi.
- Joints & Algal Fossil
There are no significant folds or fissures in the Ping Chau
Formation stratum. This indicates that the island had never
experienced rigorous structural movement. However joints and
extremely rich in the rocks provides rich opportunities for
weathering and wave erosion. During crustal movement, rocks come
under pressure from many directions. This may lead to the formation
of intercepting joints on the rocks that resemble square floor
slabs. Weathering and erosion by seawater, rainwater or even
groundwater is most probable at weak points like the joints.
Minerals containing iron oxide in the ware would deposit along the
joints and tint the edges of the "square floor slabs"
yellowish-brown.
A single rock nearby may contain fossils of algal. The algal
died when it was gradually covered by other top layer sediments.
This gave rise to slanted beddings because the direction of algal
growth crisscrossed that of the original rock bedding.
- Ripple Marks
Ripple marks are very common on the surface of rock beddings. This
is an interesting and unique feature of sedimentary rocks. Their
presence indicates the sedimentation depth was close to water
surface, and the environment was relatively calm. Ripple marks
water or wind current at that ancient time by the direction of the
ripples.
- Tufa
The white liquid that flows out is in fact calcium carbonate being
dissolved by underground water. From afar, it looks like a white
drape on the cliff. The rocks of the Ping Chau Formation are mostly
rich calcium. When underground water gets a chance to break through
rock or ground surfaces, dissolved calcium will be deposited on the
rock surface again. After prolonged evaporation, it becomes white
deposition called "tufa". This phenomenon is commonly found on rock
faces of the west coast of Tung Ping Chau.
- Verical Denudation & Slump Bedding
Vertical Denudation
Chau Mei Kok is at the northern tip of Tung Ping Chau. The stratum
enters the sea in the direction of the strike here. Wave-cut
platform made up of exposed bedrocks gradually turn into greyish
marl, and the beddings are clear and conspicuous. The rocks here
are rich in calcium. As this is a constant chemical reaction
between seawater and calcium in the rocks, the vertical denudation
caused by wave action is very obvious.
Slump Bedding
Rock beddings are normally parallel to each other in sedimentary
rocks. However, sudden "crash" of water or air current onto the
bedded sediments during the sedimentation process would disturb the
original horizontal pattern and lead to formation of slump
bedding.
To log this cache, please send the answer of the following
questions to owner:
1. When was Tung Ping Chau designated as a Country Park and
Marine Park?
2. Tung Ping Chau has a wonderful ecotour. In order to enjoy it and
to help preserve this beautiful island, what the nature code should
be followed?
3. What kind of rock for Ping Chau formation?
4. What is the "index bed" rock?
5. Where is the tallest part of the west coast of the Ping Chau
Formation?
6. At the early 70's of the last century, where was used for people
to set wild camps?
7. (Optional) Post any 2 photos of your GPSr with Tung Ping Chau
scenery in the background.
Please don't post the answer into the log.
(P.S. An EarthCache site is a special place that people can
visit to learn about a unique geoscience feature or aspect of our
Earth. The treasure is the lessons people learn about our plant
when they visit the site. Any irrelevant misleading information
will be deleted.)