Hong Kong slopes

Hong Kong’s hills are granite and volcanic rock, deeply weathered in a subtropical monsoon. The saprolite is thick; joints open; a rain band turns a slope into a debris flow. The 1972 Sau Mau Ping disaster is the classroom sentence: fill and weathered granite, not a chalk cliff.

Compare Holderness. There the budget is waves versus till. Here the budget is rain versus a weathered slope above a dense city. Same syllabus word — mass movement — different rock, different trigger, different map.

The weathering lab’s granite close-up is the mineral story. The mass-movement lab is the failure. A groyne will not save a Hong Kong slope. A cut-and-fill wall might, if the drainage is honest.

Classroom move: open mass-movement, freeze a rotational slide, then jump to the Holderness case. Ask which one the sea wrote and which one the rain wrote before you show the map.