Shifen Waterfalls #Joint #DifferentialErosion
Logging Tasks 登錄任務
Shifen Waterfall 十分瀑布
1. Predict whether the Shifen Waterfall retreats the stream or not. Explain your prediction. 請試著預測十分瀑布會不會讓河道後退。解釋你如此預測的原因。
2. Point out where are joints on the rocks? Photo it with you, your signature item, or your TB. 請指出岩石的節理在哪裡，拍下有你、你的辨識物、或你的旅行蟲跟岩石節理的照片。
3. Look the opposite side of Shifen Waterfall. There are huge rocks piling there. Can you explain why the rocks piling there? 看向十分瀑布的對面，那裏有些巨大的岩石堆積，請試著解釋為什麼這些岩石堆積在那。
Yangjingdong Falls 眼鏡洞瀑布
4. Point out river terrace. Photo it with you, your signature item, or your TB. 請辨識河階，拍下有你、你的辨識物、或你的旅行蟲與河階的照片。
5. From the introduction of waterfall's formation in the listing page, explain why there are "two" caves on it. 請試著解釋眼鏡洞瀑布為什麼會有兩個洞穴。
Log this cache "Found it" and send me your suggested answers either via Message Center. Remember, log without answers will be deleted by the geocaching guideline. Please, do not post your suggesting answers in the log except logging task 2 & 4. Hope you like this earthcache and favorite points appreciated. 你不需要等待許可，請直接線上登錄這個地質寶，並將推測的答案寄到Geocaching.com的信箱。記得，沒有答案的線上登錄會依地理尋寶守則刪除。除了登錄任務2 & 4，請不要在log中公布你的答案。希望你喜歡這個地質寶，歡迎給予FP，有任何問題請寄送訊息。
Have you ever wondered how a waterfall forms? Does it relate to tectonic activities? Why there is a waterfall? This earthcache is located Shifen, New Taipei City. This earthcache tried to explore the phenomenon in the river, Waterfall. River is so busy on its carving work on earth. Waterfalls make changes on landscape on earth. Let’s get known more about waterfalls.
What to do with this earthcache?
Expand knowledge about waterfall. Distinguish unique formation of each waterfall in Shifen. Figure out waterfall-related erosions at upper course of the river. The question is at the lowest part.
Geological properties of Shifen
Shifen is located near the headwaters of Keelung River. The formation underlying at Shifen is mainly Nankang Formation (1390) belongs to Miocene (24 Ma to 4.8 Ma). Also, the formation along the Keelung River is the layer of deposit (6060), belongs to Holocene (1.17 Ma to date).
Shifen Waterfall and Yanjingdong Falls
Shifen Waterfall is the biggest waterfall in north Taiwan. Due to its similarity with Niagara Falls, it is sometimes called “Taiwan’s Niagara Falls”. Large quantities of water flowing along the river channel fall down into plunge pool. If the weather is good with sunshine, there is chance to see rainbow hanging over the waterfall.
Yanjingdong Falls is famous with the special feature like its name in Mandarin. It looks like wearing glasses.
Formation of Waterfall
“A perpendicular or very steep descent of the water of a stream” definition from Merriam-Webster.
Waterfalls are often formed in the upper course of river. Flowing water in the river falls in steep mountain. Many waterfalls may be ephemeral and flow only during rainstorms or significant snowmelt.
Waterfalls play important role in landscape forming. Picture below shows the waterfall-related erosion of the river, including downward erosion, headward erosion, and differential erosion.
1. Differential Erosion : A classic type of waterfall may be formed with different hardness of rocks. Layer of hard rock is located at the upper position on a layer of soft rock. Downward river erosion affects both layers of rock. However, the rate of erosion on the soft rock is faster than hard rock, forming a relief.
2. Undercutting : Waters fall on the bedrocks erode the riverbed and the cliff of soft rock. The process may form potholes and undercut the cliff of soft rock.
3. Rock Collapsing: Since soft rocks are eroded, losing its supporting capacity, hard rocks may collapse down.
4. Waterfall retreats stream: After hard rocks fall down, the stream retreats. This is a kind of headward river erosion. Also a plunge pool may develop with rocks abrading the riverbed.
5. Steep valley: Vertical cliff of waterfall may develop into steep valley after million years with persistent and repeat river erosion.
From knowing the process mentioned above, waterfall-related erosions are some of important influencing factors on landscape forming on earth.
Geological properties of Shifen Waterfalls
The formation of Shifen Waterfall is unique. Shifen Waterfall endures all the process mentioned in the previous paragraph and she is a classic type of waterfall which is caused by differential erosion. The upper outcrops is slightly harder than lower one. Besides, the layer of rocks here is abundant with joints, especially lower layer.
Joint is a fracture dividing rock into two sections that moved away from each other. Joint did not involve shear displacement. Look deep into the Shifen Waterfall. You may see lots of joints on the rocks. Joints makes Shifen Waterfall different from other waterfall caused only by differential erosion.
As the previous paragraph mentioned, waterfall caused by differential erosion may have a whole layer of hard rocks on the top. The shape of the waterfall may be roughly smooth at the edge before collapsing. Shifen Waterfall indeed looks like a curtain. The peak of Shifen Waterfall is roughly smooth. However, the lower part of Shifen Waterfall has more outcrops and bumps. The reason for this unique formation is in that the upper and lower of rocks here are in a similar level of hardness. Differential erosion makes some effects but not much. Joint is the key of the formation. River erodes on the joints of the rocks. And then rocks fall down forming a relief/cliff. This becomes the unique feature of Shifen Waterfall. Picture below shows how rocks fall.
Waterfalls also have different formation reasons.
Differential Erosion
As introduced above, differential erosion is one type of the formation. The layer of hard rock may be with slight slope or horizontal. A very special kind of differential erosion is with a vertical layer of hard rock. Picture below shows different kind of differential erosion.
Hanging Valley
Hanging valley is another kind of formation of waterfall. The main stream of glacier is larger than tributary and erodes the riverbed forming a deep U-shape valley. Tributary of glacier may erode riverbed slow with less quantities of ice, forming a valley with shallow depth. After the earth becoming warm, ice melted and river formed. Waters from tributary at shallow valley flow down into the deep valley. The waterfall is formed. See the picture below.
Fault
Waterfall is related to tectonic activities indeed. Fault makes the layer of rock with a height, relief, or a cliff. The water flowing forms a waterfall. See left part in the picture below.
Obstruction on the river channel
Obstructions on the river channel may be deposits of rocks, sands, or any other stuff in the river. Water may flow on the obstruction forming a tiny waterfall. This kind of waterfall may be permanent or eroded soon. See right part in the picture above.
Geological properties of Yanjingdong Falls
The form of Yanjingdong Falls looks like a hanging valley type. However, the formation of this fall is more special than that. Hanging valley is influenced by glaciers. The geological properties of the spot here shows no evidence of glacier. Therefore, this fall is a beautiful encounter in geology. Yanjingdong Falls is located at the intersection of mainstream and tributary. The amount of water in the main stream is much more than tributary. And in that way, riverbed of mainstream is lower than the one of tributary. The basic form of a waterfall is formed.
Yanjingdong is affected not only by mainstream and tributary. River is so busy anytime. If the base level of the river lowered, the erosion will be activated again. This special phenomenon is called rejuvenation. River terrace is the evidence of rejuvenation. Let's look back to Yanjingdong Falls. You can see there are two floors in the waterfall. As the mainstream is rejuvenated, the river terrace is formed into a two-floor shape. Picture below shows river terrace and waterfall.
你曾困惑瀑布是怎麼形成的嗎?跟地層活動有關嗎?為什麼會有瀑布?這個地質寶位在新北市十分，這個地質寶試圖探索河流的現象—瀑布。河流可是非常忙碌於雕刻大地。來更深入了解瀑布吧。
這個地質寶要做什麼?
拓展瀑布的知識，區辨十分地區不同瀑布的特殊成因，瞭解跟瀑布有關的河流侵蝕作用。
十分的地質特性
十分位在基隆河源頭附近，十分下方的地層主要為南港層(1390)，屬於2400萬年前至480年前中新世，還有，基隆河畔的地層是台地堆積層(6060)，屬於117萬年前至今的全新世。
十分瀑布與眼鏡洞瀑布
十分瀑布是北台灣最大的瀑布，因為和尼加拉瓜瀑布外型相似，也被暱稱為「台灣尼加拉瓜瀑布」，巨大流量的河水通過河道墜落至水潭，若是天氣良好有陽光的話，可能在瀑布上看到彩虹。眼鏡洞瀑布的特殊特徵就像他的名字，看起來像是戴了眼鏡。
瀑布成因
「河流中垂直或是極陡的水」出自韋伯辭典的定義。
瀑布通常在河流上游形成，流水在陡峭深山中墜落，許多瀑布其實是短暫出現、或是只有在暴風雨、大量溶雪時才會出現。
瀑布在形塑地形上扮演了很重要的角色，下方圖片顯示跟瀑布有關的河流侵蝕作用，包含向下侵蝕、向源侵蝕、與差異侵蝕。
1. 差異侵蝕：瀑布典型的成因與岩石的不同硬度相關，硬岩層位在軟岩層上方，河流之向下侵蝕作用影響兩個岩層，不過軟岩層受到侵蝕速率快於硬岩層，就會形成高低落差。
2. 下切岩盤：瀑布墜落在河床岩石上，會侵蝕河床與軟岩層的崖面，這個侵蝕過程會在河床形成壺穴，也會掏空軟岩層崖面。
3. 上方岩層崩落：因為軟岩層受到侵蝕而掏空，失去了支撐能力，上方硬岩石便可能崩落下來。
4. 瀑布讓河流後退：硬岩層落下後，河流便會往後退，這是一種河流向源侵蝕作用，同時瀑布下方水潭也會因為這些落下的岩石磨蝕河床而逐漸成形。
5. 陡峭峽谷：持續數百萬年的反覆河流侵蝕作用，原先垂直的瀑布可能因為瀑布後退作用而逐漸變成陡峭的峽谷型態。
從上方提到的歷程來看，與瀑布相關的河流侵蝕作用是影響地球地表地形很重要的因素。
十分瀑布的地質特性
十分瀑布相當特別，他經歷過前一個段落中所有的瀑布成形歷程。他也確實是典型的差異侵蝕瀑布，上方岩層稍為比下方岩層來得硬一點。比較不同的是這些岩層都有很多節理，特別是下方的岩層。
節理是石頭的裂縫，讓石頭分成兩塊但沒有分離彼此，仔細看十分瀑布，你可以看到石頭上的節理，這讓十分瀑布變得獨特，不同於只受到差異侵蝕影響的瀑布。
典型成因為差異侵蝕的瀑布，岩石崩落前頂層可能為大致完整平滑的邊界，十分瀑布的頂層也是這樣，不過十分瀑布下方則有凸出與凹陷的石塊，這就是受到節理的影響，特別是下方岩塊，這個現象形成的可能原因是上方岩層與下方岩層的硬度差異不大，差異侵蝕有一些影響，但更可能受到作用於節理的侵蝕作用影響，石塊落下形成斷層/斷崖。見下方圖示。
瀑布還有許多不同的成因
差異侵蝕
上文已介紹過差異侵蝕是其中一種成因，硬岩層可能水平或是輕微傾斜。有一種比較特別的差異侵蝕型態是垂直的硬岩層。下方圖片顯示不同種類的差異侵蝕。
懸谷
懸谷是另外一種瀑布的成因，冰川的主流比支流來的更大，侵蝕作用會形成U型峽谷，冰川的支流冰量較少，可能僅能形成淺淺的峽谷，地球暖化後，冰川融化形成河流，表淺峽谷的支流往深度較大的峽谷流動，這時候瀑布就成形了。見下方圖示。
斷層
瀑布確實跟地層活動相關，斷層會造成落差、甚至斷崖，水流經過形成瀑布，見下圖左方所示。
河道上阻塞物
河道上的阻塞物可能是岩石、砂石或是其他東西的河流沉積物，水流經過河道阻塞物形成小型瀑布，這種瀑布可能是永久的，也可能快速侵蝕消退。見上方圖片右方所示。
眼鏡洞瀑布的地質特性
眼鏡洞瀑布看起來像是懸谷瀑布，眼鏡洞瀑布可要比這個成因來得更特別，懸谷的成因是因為冰川，此地並沒有冰川的證據，所以眼鏡洞瀑布其實都是因為河流侵蝕作用的結果，眼鏡洞瀑布是當地多個地質因素的美好意外，他位在主流跟支流的交會處，主流水量多於支流，河床就已經較低，這個因素讓瀑布成形有了基礎。
眼鏡洞瀑布不只受到主流/支流影響，當河流基準下降時，河流侵蝕作用會再次發生，這便是回春作用，再回頭來看眼鏡洞瀑布，你可以發現眼鏡洞瀑布有兩層，當河流發生回春作用，河階會成形，這也是兩層瀑布的成因。下方圖片所示。
Reference
地理入門
Waterfall @wiki
地質遺跡地質敏感區劃定計畫書
經濟部中央地質調查所