HKDSE Geography/M2a/Erosional Landforms

(Interlocking spurs) Interlocking spurs were not included in the curriculum and assessment guide. Only study the formation process if you have spare time. If not, simply remembering the name and appearance will do. Water in the upper course has little energy Thus it cannot remove obstacles in its path. Water flows around spurs instead of eroding them. This zig-zag movement of river flow forms interlocking spurs. Gorges Example: Wu Xia General: Downcutting along the channel forms V-shaped valleys Lateral erosion is rare Therefore, narrow, steep-sided valley - gorges - are formed. By waterfall recession: The erosive power of waterfalls is great. When water erodes the backwall of a waterfall by hydraulic action and abrasion, it retreats. A deep valley is formed. By uplift of land (less important): When land uplifts, vertical erosion occurs to form a V-shaped valley. A gorge is thus formed. Rapids Example: Bride's Pool There are alternating layers of resistant and non-resistant rocks on the riverbed The river can vertically erode the non-resistant rocks (by hydraulic action and abrasion), but not resistant rocks. This is called differential erosion. A step-like riverbed is formed. The uneven bed causes turbulent currents called rapids. Potholes Example: Near Bride's Pool At rapids, river flows over an uneven surface. When it meets and obstacle, it swirls and turbulence develops. An eddie is formed. If the riverbed is non-resistant/contains cracks, the load will drill circular hollows on the riverbed by abrasion. These are potholes. Waterfalls When a river flows down a slope, there is a strip of resistant rock (vertical, horizontal or dipping upstream) which is eroded very slowly The resistant rock in front will be eroded, and the flow of water will become more and more vertical Water will eventually fall vertically to form a waterfall Hydraulic action and abrasion undercut the base of the waterfall This creates a pool called a plunge pool Note: As there is a sudden drop in river gradient, deposition occurs at the plunge pool. Boulders can be found there. With internal processes (less important): A normal fault causes tensional force / A reverse fault causes compressional force The land slides vertically. A waterfall forms between the upper ground and the lower ground.