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Golden Beam! JinmianShan EarthCache

Hidden : 07/27/2020
Difficulty:
2.5 out of 5
Terrain:
3 out of 5

Size: Size:   other (other)

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Geocache Description:


The uniqueness of Jinmianshan

Jinmianshan is located at Neihu, Taipei. From the hiking route in the picture below, the landscape of the mountain can be discovered. The uniqueness of the mountain is that the peak of the mountain will be shining under sunshine. High quartz content in rocks is the reason for this uniqueness. It reflects the sunshine like golden beam. The quartz contents range up to 83% from an old investigation. And fossils can be found at the area of Jinmianshan.

What to do with this earthcache?

Expand knowledge of rock types in the area of Jinmianshan, differentiating sandstone and shale. Recognize sedimentary layer in Taipei Basin. Discover and classify the fossil. Observe the shape of mountain and make some classification.

Task One: recognize rocks, fossils, and Formations

Geological properties at Jinmianshan

In geology, there are two main underlying Formations at the area of Jinmianshan, Mushan Formation and Wuchihshan Formation. Wuchihshan Formation and Mushan Formation are sedimentary layers. Wuchihshan Formation was formed earlier than Mushan Formation. Hence, Wuchihshan Formation is under Mushan Formation.

Wuchihshan Formation belongs to Oligocene. And it was formed at 30 MA to 23 MA before. It is the oldest discovered formation in Taipei Basin. The main rock type in this layer is sandstone.

Mushan Formation was formed from late Oligocene to Miocene, about 24 MA to 22MA before. The main rock types are sandstone and shale. Coal mining activity was lively in Taipei and New Taipei city. Mushan Formation has a layer of Coal Formation. It was the key point to the coal mining industry in early Taiwan.

Along the Jinmianshan hiking trail, sedimentary rocks like sandstone and shale can be observed.

Shale is a fine-grained, clastic sedimentary rock. Particles in the shale are much smaller than sandstone. It is characterized by fissility, which means the ability to split along flat planes of the weakness. Tactile feeling (depends on different people) of shale is less grained.

Sandstone is clastic sedimentary rock mainly composed of sand-sized (0.0625 to 2 mm) particles. Most sandstone is composed of quartz, feldspar and lithic fragment. They have good resistance to weathering process. Tactile feeling of sandstone is more grained due to sand-sized particle. Feeling, however, depends on different people.

Fossil on the peak of Jinmianshan

From the analysis of Formations, the composition of rocks, and the discovered fossils, Wuchihshan Formation and Mushan Formation are supposed to be formed at the environment of underwater. Some fossils can be discovered at the peak of Jinmianshan. They may be the evidence to the assumption.

Let’s expand some knowledge of body fossil and trace fossil.

Body fossil are the remains of the body parts of the ancient creatures. It presents with actual bones and figures. On the other side, trace fossil provides the evidence of the movements or activities of the ancient creatures by presenting traces, footprints, boring, or caves. Body fossil tells the figure of creatures ever in the world and trace fossil tells how the creatures ever living in the world.

Examples from the picture above demonstrate body fossil and trace fossil.

Body Fossil

The ancient creatures of the body fossil are Nautilus (left) and sea urchin (right).

Trace Fossil

The left and middle example of trace fossil show the movement on the surface of sediment and dwelling cave respectively. They have some evidences for activities underwater or at littoral zone. The right example of trace fossil is footprint and shows the movement on the land.

 

Task Two: knowing the types of mountain

Jinmianshan’s height is 258 meters. It was formed into a mountain by some geological activities. Let’s expand knowing about the formation and classification of mountain from this earthcache. Then, classify the possible formation for Jinmianshan.

Classification of mountain

Generally, mountain can be classified as fault-block mountain, fold mountain, volcanic mountain, and dome mountain. Some mountains are classified by other geological forming reason, such as plateau mountains, uplifted passive margin, upwarped mountain, and hotspot mountains. All the mountains on Earth are shaped by erosion and weathering.

Fault-Block Mountain

Fault-Block Mountain is related to fault. From the picture below, rocks endure pressures. As the pressures grow, rocks would break along the fault. Blocks get pushed up or down. Then Fault-Block Mountain is formed due to the drop caused by the drop between these blocks.

Fault-Block Mountain can be massive. However, they are not as big as fold mountains. This might be the geological processes of fault are in a relatively smaller scale and involve less pressure. They are not easy to recognized while lacking the information of faults or geological activities. From their appearance, they generally have a steep side and a side in slow slope.

Fold Mountain

Fold Mountain is related to orogeny. Plates collide each other at the convergent boundary. If they are not in similar density, plate with high density would sink down and the other would rise due to lower density. On the other hand, plates with similar density collide each other causing no one sinking. The collision generates upward force. Lithospheres underlying crumble and appear to be folded. Then, the mountains are shaped by erosion effect. See the picture below.

Fold Mountains do not occur at the tectonic edges. Instead, the fold process can take place inside a tectonic plate. Fold process occurs at the collision of plates whether they are in similar density or not. Fold Mountains are usually massive due to tectonic activity. It can be over thousands of kilometers. Fold Mountains are common on Earth, like the Himalaya or the Alps.

Volcanic Mountain

Volcano is a type of mountain. The reason for the volcanic mountain is that magma rises up through the crater. They can be eruptive or flowing slowly. Volcanic lava cools down, solidifying and piling up into a mountain. Fuji Mountain is a classic example of volcanic mountain. Datun Mountain in Taipei is also a good example of volcanic mountain.

Dome Mountain

Dome Mountain is also related to magma. But it is not volcano. Magma sometimes is accumulated underneath the surface. The magma cools down and solidifies under Erath surface. It is often harder than surrounding rocks. It is exposed by differential erosion effect. Half Dome in the Sierra Nevada Range is a good example of dome mountain. 

金面山的獨特處

金面山位在台北市內湖區,由路線圖可瞭解山形樣態。金面山獨特的地方是山頂在陽光照射下會閃閃發光,這是因為金面山的岩石含有大量石英,反射陽光而造成了山頂發射金光的景象,根據一篇調查顯示金面山地區砂岩中石英含量高達百分之83,而且在金面山可以發現化石。

這個地質寶要做什麼?

認識金面山的岩石類型與組成,辨別砂岩與頁岩、辨認台北盆地的沉積地層、發現與分類化石,觀察山的形狀與嘗試分類。

任務一:認識岩石、化石與地層

金面山的地質特性

地質學上,金面山的組成有兩個不同世代的地層,分別是木山層與五指山層。五指山層和木山層為沈積地層,五指山層形成年代早於木山層,所以五指山層位在木山層下方。

五指山層屬於漸新世,約成型於3000萬至2300萬年前,是台北盆地發現到最古老的地層,主要組成為砂岩。

木山層成型於漸新世晚期至中新世,大約是2400萬至2200萬年前,主要岩石類型為砂岩與頁岩。開採煤礦活動曾經在台北與新北很活躍,木山層中有一層是煤礦層,是台灣早期煤礦工業的關鍵。

沿著金面山步道,可以沿路觀察到砂岩與頁岩等沈積岩。

頁岩為細緻紋理、碎屑固結沈積岩,組成頁岩的顆粒物小於砂岩,頁岩的特徵是可裂性,即是沿著平面的脆弱區域碎裂之能力,頁岩的觸感比較不粗糙。

砂岩是碎屑固結沈積岩,主要由沙子大小(0.0625-2釐米)的顆粒物組成,主要組成物為石英、長石與岩屑,他們有比較強的抗風化能力,砂岩的觸感因為沙子大小的顆粒物,所以比較粗糙,不過每個人的感覺都不同。

金面山上的化石

透過地層分析、岩石組成、以及發掘到的化石,五指山層與木山層應為濱海沈積相地層。在金面山山頂上可找到一些化石,可能證明地層形成的當時環境。

首先,來認識實體化石和生痕化石的不同。實體化石保留遠古生物的部分身體,以骨頭、身軀呈現,另一方面,生痕化石則提供遠古生物活動的證據,以痕跡、足跡、鑿洞、或是洞穴呈現。實體化石透露遠古生物的身形,而生痕化石則透露遠古生物曾在世界上如何生活。

上方圖片中展示實體化石與生痕化石。

實體化石

實體化石圖例中的遠古生物為鸚鵡螺 (左) 與海膽 (右)。

生痕化石

生痕化石圖例呈現生物在沉積物表層移動的痕跡 (左) 與鑿洞穴 (中間),他們提供水底活動或是濱海地區的一些證據。右方的生痕化石圖例為足跡,顯現生物曾在陸地上移動。

 

任務二:認識山的類型

金面山海拔高度258公尺,受到該區域地質活動的影響而形成山的樣子。在這個地質寶中來認識一下山的成因與類型,藉此來區分金面山屬於哪一種類型。

山的類型

山的形成大致有四種主要類型,斷塊山、褶曲山、火山、穹丘山,依不同的地質活動,還有其他的分類類型,像是高原山、上升大陸邊緣、upwarped mountain、hotspot mountain。地球上所有的山都受到侵蝕與風化的作用塑形。

一、斷塊山

斷塊山跟斷層有關。下方圖示中地質活動持續加強壓力,讓岩石受力後沿著斷層出現斷裂,兩側石塊上升或是下降,形成高低落差的斷塊山。

斷塊山的大小可以很驚人,不過可能不像褶曲山的規模那樣龐大,或許跟受力時的規模與力道比較小相關。若是不知道斷層帶、地層活動的話,其實並不容易辨識斷塊山,從外觀來看,斷塊山通常一側比較陡峭,另一側比較平緩。

二、褶曲山

褶曲山與板塊造山運動相關。大陸板塊聚合相撞時,若兩者的密度不同,較重的板塊會往下沉,較輕的往上,若是兩者的密度差不多,則板塊沒有往下沉,相撞處會產生向上的力量,兩者的岩石圈開始出現皺褶,像是下圖示意。褶曲的岩層受到侵蝕作用後逐漸開始出現明顯的山型。

褶曲山不會出現於地質構造邊緣,褶曲作用可能會在板塊內部出現,無論板塊的密度,板塊相撞就可能出現地層褶曲。因為與板塊活動有關,褶曲山通常相當巨大,可能蔓延數千公里,褶曲山是地球上常見之山,如喜馬拉雅山或是阿爾卑斯山等。

三、火山

火山也是其中一種類型的山。火山成因是地底岩漿上升,冒出地表後從火山口成為熔岩流出或噴發,冷卻成岩後堆積成山,富士山便是典型的火山類型山脈,台北的大屯山也是火山類型山脈的好例子。

四、穹丘

穹丘山也跟地底岩漿有關,但本質上與火山並不相同。有時候,地底岩漿堆積在地表下方,這些岩漿沒有抵達地表,在地底下冷卻成岩,對比周遭岩石硬度,火山穹丘中的岩石比較硬,經過差異侵蝕後裸露出來,內華達山脈的半圓丘便是穹丘山脈的例子。

 

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Question:

Task One: recognize rocks, fossils, and Formations

Waypoint I: beginning of the hiking trail

1.1 Compare the rocks at A and B. Are they the same type of rock? 1.2 Interpret your answer.

Waypoint II: the peak of Jinmianshan

        

Fossils can be discovered at the area. Compare the fossil in area C and the picture demonstrated in the listing page.

2.1 What's the possilble environment when this fossil was formed? (Land or Underwater)

2.2 What's the type of the base rock of the fossil? Interpret your obsevation.

Wuchishan and Mushan Formation are sedimentary layers under Taipei. The main rock in Wuchihshan Formation is sandstone. The main rocks in Mushan Formation are sandstone and shale. Wuchihshan Formation is older than Mushan Formation. Hence, Wuchihshan Formation should be under Mushan Formation. Base on the observation on rocks at wayppoint I & II.

3.1 The possible Formation at the peak of Jinmianshan (waypoint II) is Mushan Formation. (True/False) 

3.2 The possible Formation at the peak of Jinmianshan is the younger one. (True/False)

3.3 Explain why or why not in question 3.2.

Task Two: knowing the types of mountain

4.1 What’s the possible type of Jinmianshan?

4.2 Interpret your classification.

5. Explore the peak of Jinmianshan. Photo it with you, your identity items, or your GPS device.

Log this cache "Found it" and send me your suggested answers either via my profile, or via geocaching.com (Message Center) messaging, and I will contact you if there is a problem.

問題:

任務一:認識岩石、化石與地層

地點一:步道前段

1.1 比較A處和B處的岩石,請問他們是相同類型的岩石嗎? 1.2解釋你的判斷。

地點二:金面山山頂

    

此處可以找到化石,請比對C和寶頁中的化石圖片。

2.1 區域C的化石成形時,可能是怎樣的環境? (陸地或水下)

2.2 看看四周,化石的岩體可能是什麼類型的岩石? 解釋你的觀察?

五指山層與木山層都是台北的沉積層。五指山層則以砂岩為主,木山層為砂岩與頁岩為主。五指山層比較古老,位在木山層下方。

根據waypoint I & II的岩石觀察。

3.1 金面山頂(地點二)的地層是木山層? (True/False)

3.2 山頂的地層年代比較年輕。(True/False)

3.3 解釋3.2的判斷。

任務二:認識山的類型

4.1 金面山比較可能是哪一種類型的山?

4.2 解釋你的答案。

5. 探索金面山山頂,與你、你的代表物、或GPS拍照。

線上登錄這個地質寶,請寄到Geocaching.com的信箱,若有什麼問題,會再回覆給你。

 

Reference

內湖清代採石場 from wiki

Mountain Formation from wiki

The main types of mountains by Mihai Andrei  February 15, 2019 in Feature Post, Geology, Science ABC

Sandstone from wiki

Shale form wiki

Additional Hints (Decrypt)

Gjb-fgntr rnegupnpur

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)