Do you ever walk on the riverbed? Is it smooth or rocky? The earthcache is near the headwaters of Keelung River and is located at Dahua, New Taipei City. This earthcache tried to explore a special phenomenon about riverbed, Potholes. Activities of the mother earth on the riverbed are much more active than heuristic thinking does. Let’s explore the erosion of the river on the mountain and rocks.
What to do with this earthcache?
Explore rocks and the riverbed on the source of Keelung River. Expand knowledge about the formation and structure of potholes. Observe the riverbed to figure the water erosion of Keelung River in the deep mountain of Taiwan.
登錄任務 Logging Tasks
1. Find at least two types of pothole introduced in the listing page. Picture them with you, personal item or your TB. Post it in the log or sent to the owner. 找到至少兩種地質寶頁面中介紹的壺穴類型。請你、你的代表物、或是你的旅行蟲跟壺穴拍照。附在log或寄給寶主。
2. Go inside a pothole to observe and experience the texture of the wall. Describe your observation. Bumps, Smooth, Muddy, or Other? Explain why is the texture. 走進一個壺穴中觀察和感受壺穴的牆面質地，請描述你的觀察，崎嶇的、平滑的、泥濘的、或是其他質地？請解釋為什麼是這種質地。
3 Do you find isolated rock like an island in the river? Explain why the rock isolated from others. 你有找到河流中與其他石塊隔離開來像是孤島的石塊嗎？請解釋為什麼孤島石塊會和其他石塊隔離開來。
4. At waypoint (bridge), the speed of water current is different between left and right side (photo below). Which side can you find obvious eddies? Try to predict whether potholes are formed or not. Explain your prediction. 在橋(waypoint )的地方，見下圖，水流速度左側與右側不同。請問在哪一側可以找到明顯的漩渦？試著預測看看這一側岩體會不會形成壺穴。請解釋你如此預測的原因。
5. Observe and distinguish the terrace. Estimate how many times and meters it drops? 觀察和區辨河階地形，估計看看河流下降了幾次和多少高度。
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 1. 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1，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。
Geological properties of Dahua
Dahua Pothole is located in the valley near headwaters of Keelung River.
The formation underlying at Dahua is mainly Nankang Formation (1390 & 1395) belongs to Miocene (24 Ma to 4.8 Ma). Outcrops at Dahua Pothole are sandstone in color gray and azure. Outcrops enduring weathering effect are in slightly changed into color brown and light yellow.
The water level of Keelung River has been changed due to the Taipei Basin and Sanchiao Fault. Frequent tectonic activities occurred at north Taiwan and uplift the ground into mountains. Unique geological process of north Taiwan changed the erosion at Dahua. Erosion rate is different at different period. Dahua’s outcrop of the riverbed may be affected with rejuvenation of the stream erosion. River terrace is obvious at the area.
Dahua Pothole
Definition of pothole in earth science is “a smooth, bowl-shaped or cylindrical hollow, generally deeper than wide, found carved into the rocky bed of a watercourse”. (cited from wiki)
Some influencing factors on potholes at Dahua include rainfall, stream gradient, mass wasting, joints, and rejuvenation. The conditions mentioned before make potholes at Dahua different from other potholes in Taiwan. Potholes at Dahua are bigger and more complicated. Moreover, it can be observed the evolution of earth changing at north Taiwan through investigating potholes at Dahua.
Classification of Potholes
Based on theories, there are different ways to classify potholes. Classification in this earthcache based on the form and evolution of potholes (Tschang, 1957).
There are many different shapes of potholes. Strong erosion of the river at the same rock lying on the riverbed made more complex types of potholes. Single potholes are more likely located at shore. Compound potholes are close to the river, which are influenced more by progressive erosion.
Single Pothole
Single pothole is a bowl-like hole on the rock lying on the riverbed. This is the typical shape of pothole. These types of potholes are the initial stage of pothole. It may be smaller, shallower in depth.
Linked Pothole
Linked Pothole is another classic type. Linked pothole has a Calabash-shape or Hulu-shape (in Chinese). Two potholes are formed close to each other at first. While the erosion enlarges a pothole, the other one is also enlarged. Two potholes finally evolve into a calabash-shape like hole. If the potholes are formed at different time, the linked pothole might be shallow and deep respectively in longitudinal profile.
Compound Pothole
Compound pothole is a complex type. It can be view as a new pothole formed in an old pothole. It takes time and needs active water erosion. This is more frequent showing at upper ranches or bending streams. Besides, the effect of erosion may be different at different time. It might be related to changes in slope of the river, rock composition, or the climate, etc.
Linked Compound Pothole
Linked compound pothole is more complex type. It might be in diverse shapes. Two or more compound potholes were enlarged as the time flies. They would be linked together gradually into a huge and complex shape like pothole. The formation process takes a long time. The result of the water erosion made rock formations in very unique shape and structure. Some rock even looks like in melting.
See the demonstrating picture below.
Formation of Potholes
Potholes at Dahua are holes on the outcrops along the river or on the riverbed. Flowing water is the main influencing factor. Water currents run on the river channel. Unsmooth riverbed cause eddies. Some stones piled on the riverbed or rocks with angles change the direction of current. Small stones carried by water current may collide the rocks on the riverbed. This kind of water erosion affects on the weak zone of the rocks. The hardness of the rock may differ with composition or weak area like joint. Water current, small stones, and hardness of rocks make shallow hole on the surface of host rock.
Then, the influencing factors continue the erosion on initial potholes. The enlargement of pothole is influenced by a kind of unique erosion, evorsion. Evorsion is a special way of erosion, especially on potholes. Evorsion means eddies caused by unsmooth riverbed carry small stones in the shallow hole and the water current erodes the rocks. Evorsion in the pothole would enlarge and deepen the hole. The process takes time and makes potholes in different forms. Strong evorsion can even erode on the wall of a pothole till a new hole on it. Water can go through from the hole. Make the rock formation more unique then.
See the picture below to know the potholes in different stages.
Water erosion of Keelung River
Stream erosion is both downward, lateral, and headward. These ways of stream erosion all occur at the area of Dahua, which is near the headwater of Keelung River. Climate, vegetative cover, topography, tectonic, and development are influencing factors on stream erosion. The erosion will approach a stable situation while all the influencing factors keep still. However, tectonic activities of north Taiwan and topography of Keelung River changes at different period. The exit of the Keelung River had been changed. The base level of the river also changed. It had been blocked by Taipei Basin and released into the sea with Tamsui River through Sanchiao Fault. Tectonic activities and some faults nearby at New Taipei City also changed the height of Dahua. The stream erosion of Keelung River at Dahua also changes.
See the picture below to know the result of the erosion and rejuvenation.
Rejuvenation and River Terrace
Rejuvenation means that stream erosion may be rejuvenated in response to a lowering of river’s base level. A terrace is the product of river erosion and rejuvenation. The newly formed terrace becomes deeper and formed into valley with flowing water. Then, this valley is widened through lateral erosion. The process continues for a long time into a floodplain. If the river’s base level changes with the influencing factors mentioned above, the erosion may be rejuvenated and begins to erode downward again. Repeated rejuvenation results in a step like profile around the river. Therefore, river terrace is one of the evidence of river rejuvenation.
你曾試著走在河床上嗎？河床是平滑的還是充滿岩石？這個地質寶位於新北市大華且接近基隆河源頭處。此地質寶試著探索河床上的特殊地質現象—壺穴。地球在河床上的活動比第一印象還要多很多。一起來探索河流對山脈與岩石的侵蝕作用。
這個地質寶要做什麼？
探索基隆河上游的河床與岩石。拓展壺穴地形知識，瞭解壺穴的結構與形成原因。觀察基隆河河床，瞭解北台灣深山中的河流侵蝕作用。
大華地質特性
大華壺穴位在基隆河源頭的峽谷，大華的主要地層為南港層，屬於2400萬年前至480年前中新世。露出的岩層主要為灰色與深藍色砂岩，風化作用影響下表層顏色可能轉變為棕色與淺黃色。
基隆河水面變化曾經受到台北盆地與山腳斷層影響。北台灣頻繁的地質活動推升地層形成山脈，北台灣獨特的地質特性改變大華地區的侵蝕作用，在不同時期大華地區的侵蝕速率有所不同，大華露出岩層可能受到河流侵蝕的回春作用影響，河階地形在這個區域很明顯。
大華壺穴
壺穴的地球科學定義為「刻畫於水流域之岩石石盤的平滑、碗型、空心筒狀洞穴，一般來說比較深不是比較寬」(from wiki)
大華壺穴的影響因子包含降雨量、河流坡度、土石流失、節理與回春作用，前述影響因子讓大華壺穴與台灣其他地區的壺穴明顯不同，大華地區的壺穴比較大且比較複雜，還能夠透過觀察大華壺穴來觀察北台灣地質變化的演進。
壺穴的分類
根據不同的理論可以將壺穴做不同的分類，此地質寶中的分類系統依據形狀與壺穴演變。
壺穴有不同的形狀，河流強力的侵蝕作用發生於河床上的同一塊岩盤，能形成更複雜的壺穴。單一壺穴比較可能分佈座落在河岸上的岩石，複成壺穴分佈上則會比較接近河流，這類壺穴會受到更強力的侵蝕影響。
單一壺穴
單一壺穴是河床岩石上的一個碗形洞，這是典型的壺穴形狀。這種類型壺穴是壺穴地形發展的第一階段，可能會比較小、比較淺。
聯合壺穴
聯合壺穴則是另一種典型，聯合壺穴形似葫蘆，形成時兩個壺穴靠近彼此，接著壺穴持續受到侵蝕作用影響擴大，最後兩個壺穴相連在一起成為葫蘆狀的洞。若是兩個壺穴在不同時期形成，則該聯合壺穴剖面圖看來會一深一淺。
複成壺穴
複成壺穴為比較複雜的類型，可以理解為新的壺穴在舊的壺穴中形成。這個過程需要很長的時間，還得要有很強力的水流侵蝕作用，較常出現在河流上端分之或是彎曲的水流處，此外，不同時期可能會用有不同強度的侵蝕作用，這就牽涉到河流坡度的改變、岩石組成、或是氣候等等因素。
聯合複成壺穴
聯合複成壺穴則是更複雜的類型，這種壺穴會有多樣的形狀，兩個或是更多複成壺穴隨著時間而增大，他們會逐漸相連成為一個大型與複雜的壺穴，形成過程需要很久的時間，水流侵蝕作用的結果讓岩石成為極為特殊的形狀與結構，有些岩石甚至看起來像是在融化。
參閱下方圖示。
壺穴形成過程
大華地區的壺穴為露出岩體的洞，沿著河岸分佈或是分佈在河床上。水流是主要的影響因素，水流沿著河道流動，不平緩的河床產生漩渦，堆疊在河床上的石頭形成角度而改變水流的方向，順著水流牽動的石塊撞擊河床岩體，這類型的水流侵蝕作用發生於岩石的脆弱區域，像是岩石組成可能會有不同的硬度，或像是節理之類的岩石脆弱區域，前述之水流、礫石、和岩石硬度在岩體表面形成淺淺的坑洞。
接下來，侵蝕作用持續影響初期壺穴。擴大壺穴則涉及到一種獨特的侵蝕作用—渦蝕作用。渦蝕作用是相當特別的侵蝕作用形式，特別是影響壺穴上的效果。渦蝕作用是指因不平順的河床所造成的漩渦牽引著礫石，在表淺坑洞中持續侵蝕著岩石。壺穴中的渦蝕作用會擴大與加深壺穴，這個過程耗時且會讓壺穴變成各種不同的樣式。強力的渦蝕作用更可能鑿破壺穴岩壁讓水流通過，進而形成更特殊的岩體。參閱下方圖示理解壺穴的不同階段。
基隆河的侵蝕作用
河流侵蝕作用可有加深、加寬與向源侵蝕形式，這些不同類型的河流侵蝕都在大華區域發生，畢竟大華就位在基隆河的源頭處，氣候、地表植被、地形、地殼活動、和地形發展都是影響河流侵蝕作用的因素。當這些因素維持不變的時候，河流侵蝕作用也可以達到平衡，不過，北台灣的地殼活動和基隆河的地形變化在不同時期有所不同，基隆河的出口就曾經改變過，河流的基本水平面就跟著改變，基隆河出口曾經隨著台北盆地而阻塞過，也曾隨著山腳斷層陷落而跟著淡水河釋放進入海洋，還有些地層活動與斷層改變了大華地區的海拔高度，因此大華地區基隆河的水流侵蝕作用也會跟著改變。
參閱下方圖示瞭解地殼上升和水平面下降對侵蝕作用的影響結果。
回春作用與河階地形
回春作用意指河流侵蝕作用隨著河流水平面下降而重啟。河階地形則是河流侵蝕作用與回春作用的產物。新形成的河階受到水流影響而持續變深成為峽谷，然後峽谷透過側向侵蝕作用而變寬，這個過程持續相當久後形成氾濫平原，然而若是河流水平面因為前述影響因素而改變，侵蝕作用可能重啟並再次發生河流侵蝕之加深作用。重複的回春作用導致側面圖來看河床形似階梯。所以，河階地形是回春作用的證據之一。
Public Transportation
Train to Dahua Station. Along the Reference point to the earthcache. 搭車到大華車站，沿著參照點走到地質寶處
Beware that there is no signal. Memorize the question while you are with the internet. 注意壺穴附近沒有訊號，趁還有網路時記下問題
Thanks for your visit. 感謝您的拜訪
Reference
地質遺跡地質敏感區劃定計畫書
Tschang, H.L. (1957) Potholes in the River Beds of North Taiwan.
經濟部中央地質調查所 地質資料整合查詢
Pothole from wiki
Rejuvenation from wiki
River terrace from wiki