Introduction to Elephant Mountain and Six Giant Rock
Elephant Mountain, or Xiangshan (象山), is one of the most iconic natural landmarks in Taipei, Taiwan. Located in the Xinyi District, it is part of the Four Beasts Mountains, offering breathtaking views of Taipei’s skyline, including the famed Taipei 101. While known for its hiking trails and scenic vistas, Elephant Mountain also features fascinating geological formations.
There is a fascinating rock formation, Six Giant Rock, along the trail. The rock strata of elephant mountain link to Taliao layer and Shihti layer, formed by the sediments scoured out from HwaNan land mass around 22-18 million years ago. The giant rock boulders are sedimentary rock from the ancient strata. These rocks are sandstone with stratification(bedding) of tiny grains of sediments and with less jointing cracks.
Stratifications and Joints on the Ridge of Elephant Mountain
After passing the six giant rock, you will visit a huge rock cliff (posted coordination) with three Mandarin characters(象山崗) on it, which means the ridge of elephant mountain. The front of the rock cliff shows the stratification/bedding of sand grains in 40 degrees. Few steps up on stairs to the backside of the rock cliff, clear jointing shows the trace of weathering.
What to do with this earthcache?
This earthcache will lead you exploring the geological facet of elephant mountain. Concept of joints, a type of physical weathering, and stratification, a fundamental feature of sedimentary rocks, will be explained and illustrated.
Understanding Joints in Rocks
Joints are natural fractures or separations in rocks where there has been no significant movement on either side. They form due to stress in the Earth’s crust, caused by factors such as cooling, contraction, or tectonic forces. These cracks allow water and other agents of physical weathering to penetrate, contributing to the breakdown of rocks over time.
Joints can be classified according to their origin, like tectonics, hydraulics, exfoliation, unloading (release), or cooling.
Tectonic joints often reflect local tectonic stresses associated with local folding and faulting.
Hydraulic joints formed when fluid in the pores of rocks elevates the pressure and causes cracks in response to those pressures.
Unloading joints (release joints) are joints formed when uplifting and erosion removes overlying rocks thereby reducing the comprehensive load and allowing rocks to expand laterally.
Exfoliation joints may be a special case of uploading joints. The surface of rocks weathered into the joint plane due to the decrease in temperature and pressure. These joints can be flat, irregular, or curved.
Cooling joints are formed by cooling of hot rock mass. For example, lava cooling into columnar joints with typically hexagon shape.
Stratification in Sedimentary Rocks
Stratification, also known as bedding, is a fundamental feature of sedimentary rocks, characterized by the arrangement of sediments into distinct layers or strata. Each layer represents a specific depositional event, with variations in grain size, composition, and color indicating changes in environmental conditions over time.
At Elephant Mountain, the visible stratification in sedimentary rocks provides a glimpse into the region’s ancient depositional environments, enriching the understanding of its geological history. Like, Shihti Layer, one main layer of the elephant mountain, is featured with rich coal deposits, making rock here with deep color or even black lines in the rock.
Logging Tasks
Observe joints behind the rock cliff at the posted coordination.
1. What shapes of joints do you recognize? (liner/curved/irregular)
2. Which type of jointing do you classify those joints on the rock cliff? (tectonic/hydraulic/unloading/exfoliation/cooling) Explain the answer and observation.
3. Do you recognize stratification/bedding in sedimentary rocks on the cliff? Would the bedding affect the joints here? Explain your thoughts.
(Optional) 4. Photo joints on the Ridge of Elephant Mountain with you, your identifying item, or your GPS device.
Log this cache "Found it" and send me your suggested answers via geocaching.com messaging. Please, do not post your answers in the log, except task 4. Hope you like this earthcache. Favorite Point Appreciated.
象山與六巨石簡介
象山，或稱為「象山」（Xiangshan, 象山），是台灣台北市最具代表性的自然地標之一。位於信義區的象山屬於四獸山系，以壯麗的台北天際線（包括著名的台北101）視野聞名。 除了登山步道和迷人的景觀外，象山還有著迷人的地質特徵。
沿著登山步道，有一個引人入勝的地質構造—六巨石。象山的岩層屬於大寮層和石底層，是約 2200 萬到 1800 萬年前，由華南陸塊侵蝕的沉積物而形成。這些巨石為古代地層中的砂岩，帶有細小沉積物的層理（或稱層狀構造），並伴隨少量的節理裂縫。
象山山脊上的層理與節理
越過六巨石後，您將抵達一處巨大岩壁（座標位置），岩壁上刻有「象山崗」，即象山山脊。岩壁正面顯示出砂粒層理/層狀構造，傾斜角度約為 40 度。再向上幾級階梯至岩壁背面，可以清楚地觀察到節理裂縫和風化痕跡。
這個地質寶要做什麼？
這個地質寶將帶領您探索象山的地質面貌，深入了解岩石節理（物理風化的一種形式）和層理（沉積岩的基本特徵）的概念及說明。
認識岩石中的節理
節理是岩石中的天然裂縫，裂縫兩側並無顯著移動。節理的形成源於地殼內部的應力，例如冷卻、收縮或構造作用。這些裂縫允許水及其他物理風化因子滲透，進而促進岩石的分解過程。
節理可根據其成因分類，例如構造節理、水壓節理、卸載節理、剝離節理或冷卻節理。
• 構造節理：反映局部摺皺或斷層活動引起的構造應力。
• 水壓節理：由岩石孔隙中的流體壓力升高所致，導致裂縫生成。
• 卸載節理（釋放節理）：由於地殼抬升與侵蝕移除了上覆岩層，減少了壓力，岩石因而側向擴張形成裂縫。
• 剝離節理：通常屬於卸載節理的一種特殊情況，岩石表面因溫度與壓力降低而在節理平面上風化，這些節理可能呈現平坦、不規則或弧形。
• 冷卻節理：由熔岩或熱岩體冷卻形成，例如六邊形的柱狀節理。
沉積岩的層理特徵
層理（或稱層狀構造）是沉積岩的基本特徵，指沉積物以明顯的層次排列。每一層代表一次特定的沉積事件，層次中的顆粒大小、組成和顏色的變化，反映了不斷變化的環境條件。
在象山，沉積岩清晰可見的層理結構展現了該地區古代沉積環境的面貌，豐富了我們對其地質歷史的認識。例如，象山的主要地層之一：石底層，蘊含豐富的煤層，讓此處的岩石中較為深色、或甚至黑色的條紋。
登錄任務
觀察公布點岩壁背面的節理。
1. 您觀察到節理是什麼形狀？（線狀/曲線狀/不規則）
2. 您如何分類這些岩壁上的節理類型？(結構節理/水壓節理/卸載節理/剝離節理/冷卻節理)請解釋您的答案及觀察結果。
3. 您有觀察到沈積岩的層理嗎？請問層理會不會影響此處的節理？如何影響？
4. (自由選擇)與象山崗的節理合照，照片中包含節理與您、辨識物品或 GPS 裝置。
登錄此地質寶為「found it」並透過 geocaching.com 訊息發送給我您建議的答案。除了任務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。
reference
unloading joints by Marli Miller