Human Nature EarthCache

Human Nature: the influence of human activity on landscape formation and some of its consequences.

This earth cache takes you for a hike first to a small hill in between Tin Shui Wan and Yuen Long (the listed coordinates), and after that to Nam Shang Wai for a close-up look. It will take you at least four hours, probably more as there is so much more to see especially in the Nam Shang Wai area.

Introduction

We humans are shaping our planet the way we want it. We remove mountains, fill up valleys, and destroy a lot of our natural environment in the process.

Human activity also creates important habitats - habitats without which certain species would become extinct. One of the most common examples is of course the house fly: a fly so adapted to live in human houses, that it can not survive outside of it.

But on a larger scale human activity shapes the planet too. The coordinates take you to the summit of a hill, yet the cache is about the coastlines and human influence on that. This hill provides a great viewpoint of the natural shorelines north of Yuen Long, and the immense human influence on this shoreline, which is best viewed from above. It is a coastline many people would call "natural" while in reality it is largely "man-made". But then aren't humans part of nature? So isn't man-made actual natural? But this is an earth cache, not a biology or philosophy lesson, so let's go down to earth and science.

The photos of the views - links at the bottom of this page - are poor thanks to the foggy weather; better photos (taken on a clear day) would be appreciated!

Note on wheelchair access: Stage 2 (Nam Shang Wai) is mostly wheelchair accessible. Stage 1 (Kai Shan) is not. Wheelchair users wanting to log this cache can do most of it, and may just log the for them accessible parts. Note that the small ferry at WP1 can not take wheelchairs (baby strollers will be possible as they can be folded and hand carried onto the vessel), wheelchair users have to use the road to access the area. You can take taxi into Nam Shang Wai.

History of the area.

About 18,000 years ago, when glaciers reached their worldwide maximum extent, the sea level was 120m lower than today. Then, as a result of natural global warming, most of the ice melted and sea levels reached modern levels about 6,000 years ago and haven't changed much since. At the time the areas that are now Yuen Long and Kam Tin were a bay. Fluvial deposits from the Kam Tin and Shan Pui rivers slowly filled up the area. This process of sedimentation where rivers meet the sea is called progradation. Even as recent as 1913 the coast line looked radically different from what we see now. Construction of gei wai (shrimp ponds) around the 1940s caused the coast line to move seaward dramatically, later large areas of these gei wai were reclaimed for construction of homes, factories and container yards.

Floodplains.

The Kam Tin and Shan Pui rivers reach the sea here, as a result the velocity of the currents drop and sediments can settle down. These sediments cause the filling up of the river channel, over time resulting in dry land which tends to be very flat and fertile, and very suitable for agriculture. The Kam Tin and Pat Shap Heung areas are also part of the floodplains.

The floodplain doesn't end at the shoreline: sediments are carried further in the sea, so the area near the river mouth is shallow, and is slowly gaining more and more sediment. As a result the natural coast line moves north. The shallow waters allow mangroves to grow, slowing down flows even more and helping the settlement of sediment.

With increasing population, the human influence on the shores increased dramatically. In the mid-1940s, vast mangrove forests were reclaimed and shrimp ponds, called gei wai, were constructed. These ponds blocked the natural water flow and caused the coast line to advance rapidly. These now mostly abandoned shrimp ponds are still visible in areas such as around the Wetland Park, Nam Sang Wai, Lut Chau, Mai Po and San Tin.

Partly due to this human intervention the Mai Po mangroves extended by an average of 11.5 meters a year between 1949 and 1969, and 3.4 meters a year between 1969 and 1987. The area of mangrove forests has trebled since the construction of the gei wai.

The gei wai provide important habitat for various water fowl. Water in shrimp ponds is shallow, and is kept free from mangroves, reeds and other water plants by the shrimp farmers. To harvest the shrimp the ponds are drained, allowing not only the farmer to easily scoop in the shrimp, but the birds to pick up the rest of the fish. That is mutually beneficial: the birds have an easy meal, and the shrimp farmer has his pond cleaned from unwanted inhabitants.

Of course nowadays most shrimp farmers have abandoned their ponds, leaving them for nature to take over. Conservation efforts are on the way to provide this unique man-made landscape, and to protect the birds that have come to rely on it.

About the cache.

This cache will take you for a walk around the area. The first point is Kai Shan, a small hill in between Tin Shui Wai and Yuen Long - this is the 3-star terrain, it is not high but the trails are rocky and unpaved. The second part is focused on Nam Shang Wai, one of the old fish pond areas famous for it's natural beauty. This is 1-1.5 star terrain, flat and paths mostly paved or have solid surface. You are advised to start at Tin Shui Wai station, from there hike up Kai Shan, and then on the other side down to Long Ping Estate. This walk takes about two hours, you may consider having lunch in Long Ping Estate before continuing. Nam Shang Wai can be done in an hour but you are strongly recommended to schedule more time for a visit to that area as there is so much more to see.

You will be able to log a few traditional caches on the way; notably GC3A76E and GC2ZCAM.

Stage 1: Kai Shan.

Kai Shan is a natural hill, providing a great vantage point over the area. This is why the earth cache is located here (the green arrow on the map below). To the north you can see gei wai, part of which are now part of the Hong Kong Wetland Park. Behind the gei wai is a large mangrove forest, on an intertidal mud flat. These areas flood at high tides, and fall dry with low tides.

An annotated aerial map of the region, GZ is at the green arrow, bottom centre.

To the east is Yuen Long Industrial Estate, on land reclaimed from former fish ponds. The protected wetland park was constructed as ecological compensation for the loss of these wetlands. And to the west lies Tin Shui Wai new town, which is partly built on land reclaimed from gei wai. Finally to the south lies Yuen Long new town, which is mostly built on established land. The construction of these towns and the industrial estate of course also changed the landscape drastically.

Under your feet the hill is made up of fine grained metasandstone from the Lok Ma Chau Formation, a sedimentary rock formation dated to the Early Carboniferous - deposited 354 to 280 million years ago (source: CEDD). At the cache location several large boulders are exposed on the hilltop, the rocks are intermittently exposed on the path as well. That you can see this type of rock here is very interesting, as it is basically the same type of rock that is in the making in Deep Bay. The current deposits there, if left untouched and covered by more material, will turn into similar rocks over the upcoming tens of millions of years.

Stage 2: Nam Shang Wai.

An excellent example of the fish pond and gei wai development. This area was very shallow sea and mangrove before settlement, now the mangroves have been pushed way out and the shallow seas have become ponds. Most of these ponds have in turn been abandoned, and are now overgrown by reeds, slowly filling up the water with dead plant material, effectively creating new land. This is a natural process, in this case seriously accelerated by human activities. Reeds can completely fill up such shallow ponds in just a few decades, creating the base for dry land.

WP1: is where you can take a small boat across the water to enter Nam Shang Wai, the fare is $5.

WP2: some fish ponds that are still in use. These ponds are seasonally drained by the farmers.

WP3: Mud flats, in the sea outside the main dike around Nam Shang Wai. Here you can witness natural land formation in progress. Every tide the water deposits another thin layer of mud. When the mud becomes high enough, mangroves can start growing there (you may see some saplings standing out). These mangroves help the continued deposit of mud, and their roots hold together the existing layers preventing the material to be washed away.

WP4: here you find yourself on a dam in between two abandoned fish ponds. This dam was made out of the local mud. Notice the difference in vegetation on the dam and in the abandoned fish ponds on either side.

WP5: here is a small fish farming community, that is still using the fish ponds to farm fish. These ponds are free from vegetation, also the surrounding dams are maintained. This is an example of how the whole area looked like in it's heydays.

WP6: a to-fu-fa stand, with a beautiful view over a fish pond. After this point the road back towards Yuen Long is less interesting. And it's the end of the cache route. From there it's a 3 km walk along the road to Yuen Long station.

Logging tasks.

• General.
  1. Water tends to move anything from large boulders to fine silt and mud. Which type of sediments do you expect to find near the river mouth, and why?
  2. How do you think do this gei wai development helps to accelerate this process?
• At the first location (Kai Shan).
  1. In between the Industrial Estate (formerly fish ponds) and the fish ponds to the north of it, you can see a village development (approx. 45° magnetic, see photo and the map above). At this location three villages are located: Shing Uk Tsuen, Ng Uk Tsuen and Tai Tseng Wai. Why would the original settlers have chosen that exact location?
  2. Have a closer look at the rocks under your feet. Describe them. Think in terms of texture, layering, colour, etc.
• At WP3: how would these mangrove forests help increasing the deposit of mud?
• At WP4: describe the difference in vegetation on the dam and next to it.

Final notes.

For more information about wetlands you are recommended to visit the nearby Hong Kong Wetland Park, with an indoor exhibit area about the ecology of wetlands and an outdoor area featuring bird watching and mangrove forests. Pui-pui the Yuen Long crocodile lives here as well. Visit their web page for information on opening hours and ticket fees.

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.