Ma Shi Chau VR Field Trip
Hello Today we are going to have a geological field trip to Ma Shi Chau. Ma Shi Chau is on the shoreline of the Tolo Channel. To get there, we actually took the bus and we arrived a village called Sam Mun Tsai New Village. Our starting point is here. You can look around. In Sam Mun Tsai Village, people live here round this region for more than a hundred years. At that time, most people were fishers and lived on a boat. So, this is the new village, people settle here in 1960s. So, we are going to start from this location and go hiking and we go to the Ma Shi Chau Geological Park. Alright, we have four students, Peter, Jay, Wincy and Joanne. Let’s go.
Although fishing is declined in recent years, the local villagers are preserving the original culture and relics. We are going through the street of the new village. Now we are at the starting point of the trail. It is 1.5 km walking for half an hour. Let’s go with me After 20 minutes walking, you can see far away Ma Shi Chau is almost there. Let’s go.
Here we go. Here is the sand bar. If the tide is lower than 1.5m, there is no problem to cross the sand bar. You can see visitors come from a small boat, maybe from the south part, that is Science Park. Hello everyone. Let’s start our geological field trip. First of all, I would like to introduce the geological history of Hong Kong. The oldest rock in Hong Kong is about in the Devonian period, about 400 million years ago. Followed by Carboniferous and Permian rocks. Here in Ma Shi Cha, this is Permian
period, the second oldest rock in Hong Kong. In the Triassic period, we don’t have any deposition. So here, these belong to early sedimentary rocks. From 160 to 140 million years, Hong Kong experienced violent movement of tectonics and the formation of volcanoes. Igneous rocks are formed during the late Jurassic and early Cretaceous period. And then followed
by the Tertiary and Quaternary sedimentary rocks formation which are the youngest. Here is a local geological map. The earliest sedimentary rocks in the Devonian Carboniferous period are distributed along the Tolo Channel. Here is the Ma Shi Chau, formed during the early Permian period, which is around 280 million years ago. Here we can see
quite a lot of mudstone, siltstone, sandstone, conglomerate. Right now, we are standing here. During this visit, we are going to walk along the south east coast of Ma Shi Chau. Here we can see the tombolo, which is over there, and also the siltstone, the shell beach, the colorful mudstone, quartz vein, differential erosion, sandstone, fault and fold, structures. Ma Shi Chau is a representative of old sedimentary rocks in Hong Kong in the Permian period about 280 million years ago. We can find that most of the rocks, as I mentioned, are sandstone, siltstone, mudstone formed by the sedimentation process. And also, we can find the marine organism and plant fossils in the siltstone and mudstone, which indicated that this has been a marine environment. So, talking about the
tectonic movement during the years, here I have an illustration of the sea level changed over the years. We can see in the early Carboniferous period, here is the deep-sea environment. A deep continental sea environment was developed during early Permian period and you can see here is represented by these fossiliferous siltstones, mudstones developed in this location. So, the sea level was changing. The late Jurassic period was represented by volcanic activities and
later on the sea level lowered down. Here Hong Kong is a part of the continental margin, the land. We can see a lot of tectonic changes have been experienced in this area in the past 400 million years. Let’s start our trip. Here we can see a sand bar, actually it is called a tombolo. A tombolo is a sand and gravel accumulated over time
under the action of the wind and wave. Because this location is a little bit narrow, so over time the sand is accumulated here and finally connected to these two lands. Let’s go to the beach. Here is called shell beach. You can
see a lot of shells accumulated on the shoreline. It is very nice to walk over the sand beach. Here we arrived the first spot, colorful mudstone. You can see around here. There are many beautiful and colorful mudstone. You can see different color. You can see the yellow brown color, the red color, white color, grey color. So I mentioned earlier, Ma Shi Chau was deposited in a tidal flat and coastal environment. So the full formation, we call the Tolo Channel formation is about 500 meters thick. Here in my hand, this is a mudstone.
The mudstone is formed by very fine particles. If you touch it, it is quite smooth, and you can easily scratch some lines and leave some marks on the surface. So this is quite a beautiful piece of mud stone. You can talk a close look.
Look at the beautiful patterns. We can take a closer look. You can clearly see the well-developed beddings, representing the deposition sequence, like here. Here at this site, we try to talk about sedimentary rocks. As I mentioned, the Ma Shi Chau site is formed by old sedimentary rocks of 280 million years ago. As talking about sedimentary rocks, we mainly distinguish them by the grain size. The mudstone, claystone or called siltstone are usually made of
fine grain materials. So relatively speaking, the sandstones are coarser, and mudstone and siltstone are smoother. If the grain size is even bigger, we call it conglomerate if the particles are rounded. Or call it breccia if they are made by angular particles. So here I try to give you some samples to illustrate them. This is a piece of
mudstone, we see around here, mainly of these are mudstone. You can easily scratch the mudstone because they are quite smooth, and they are made by very fine clay particles. Siltstone is a little bit rough, but still the sand grains are pretty fine. This is a piece of sandstone. You can see the surface is pretty rough and they appeared to be stronger as well. Here I have a big piece of sandstone here.
You can see it is intruded by quartz veins. Take a closer look at this one. This is sandstone and quartz intruded inside, very fine hairline, we call quartz veins. This is quite a unique place.
Again, we have mudstone, siltstone and sandstone all in this site. Here I see quite a big rocks. Let’s take a look. If you touch it, it's quite rough, so this is a sandstone.
You can look at this beautiful sandstones. Look at here You can see here, there is a lot of white colour crystals, actually they are quartz. When the quartz, silica (SiO2) actually infiltrated into the cracks of the sandstone and deposited it inside the crack, forming the so-called quartz veins. The quartz is relatively more resistant to weathering that's why you can see the quartz veins is pretty hard, so they're cutting through these rocks in the different places and some form very beautiful structures.
So it's just like our blood veins, so we call it quartz veins. Here is also very beautiful. Again the white coloured are quartz. Okay let's go. You can see the different colours.
A beautiful red colour. This is only because the sandstone is weathered, so when it's weathered it turns into the brown colour or even the red colour. Interesting! Here we have a very interesting phenomena called differential erosion. You can see the rocks are eroded to a different degrees. This part was fully eroded but this part is much stronger, so it remains, preserved.
So this is a layer of sedimentary rock, kind of like siltstone and sandstone. These layers are bedded, you can see the beddings and it's dipped to the East. Due to the differential erosion, we can clearly see this piece was preserved all the way to the shoreline to the sea. It's very beautiful. This part is weathered more.
This part looks like they're chertified. it is much stronger and this part is much weaker, so that cause the differences in the weathering resistance. It's very beautiful. Okay let's move on.
We continue to go along the shoreline. This place is the most beautiful. You can see again. This is a sedimentary rock, like a siltstone.
You can see that all the beddings. Can you see the beddings here? It is dipping almost vertically, this is facing the direction of the southeast. So I put the camera here. All right, you can see here, the beautiful white coloured. What it is? Let's take a look. You can easily scratch it, so they're quite soft. This is a clay stone and embedded with siltstone.
So there are different sequences embedded together, very beautiful sedimentary layers. Very soft while this part is relatively hard. The siltstone is harder than the claystone. So you can see the clay powders. You can easily scratch it and white powder, which is Kaolinite clay.
Again we see differential erosion. The softer claystone are eroded easily, leaving a recessed area. Here, the siltstone is much stronger, so they are preserved. The white colour is just like snows, very beautiful to walk on this rock. You can clearly see the siltstone is interbedded with the claystone and the mudstone.
Sedimentary rocks are layered. They have bedding. The bedding was initially parallel and horizontal. Now under the tectonic force, they are overturned.
Here we also see it very interesting sedimentary structure, which is called the fold. The fold was formed when the sediments are under the compression load. Here we can clearly see this layer and make a turn which is over here almost 90 degree, so this structure is called fold.
We see a lot of folds in this area, that means, when the sedimentary rock was formed, they're under significant tectonic movement. They are under the compression and significantly folded. Here, we also see the curve. This is the folding. Here again, you see this layer actually makes a turn. Very local foldings are presented on the site almost everywhere. Very interesting.
We can see the bedding direction is along here, there is a different angle. So that represents the fold structure. Okay let's go to find the last spot.
Alright, here is the most interesting phenomonon in the site. You can see this very special layer has oval-shaped blocks that look like sausages, or boudins, inside the whitish finer grained siltstone or mudstone. Now let me explain how these boudins/sausages were formed.
In the beginining, there were alternating layers of hard sandstone and soft mudstone/siltstone. The layers were under compression and streteching. The sandstone, being hard and more brittle, broken into a series of disconnected blocks, resembling boudins or sausages. Finally, the sandstone was pulled apart, and the softer mudstone/siltstone flowed into the gap, filling the space between the oval-shaped boudins or sausages.
This is called Boudinage structure in geological term. Or simply, we call them sausages. This is one of the most interesting parts in this site. Lets take a look, there are quite a lot of places having this kind of sausages Here as well, they are along this layer. Alright, lets go forward. This place...
In this place, I see three oval shapes, like eggs. This one was taken off, leaving a dent. Look at this place, harder sandstone is embeded in softer mudstone/siltstone. The sandstone is more resistant to wheathering. Hello! We are at the end of our field trip. Today, I’m very glad to have the wonderful afternoon with Wincy, Joanne, Peter and Jay.
Ma Shi CHau is the Hong Kong's Permian Rock garden. We saw a lot of very beautiful sedimentary rocks, quite rare in this region. Hopefully you can come to this site and could see these beautiful rocks by yourself.
Thank you very much and see you next time.
2022-04-17 04:01
