Sai Kung Hexagonal Rock Columns

Rock FormationsSai Kung, New Territories,Hong Kong

Fragile nature — stay on the trail, leave no trace.

Pale rhyolite columns rising 100 metres out of the South China Sea in Hong Kong, cooled inside a supervolcano's caldera 140 million years ago.

September·Peaking now

Jun–Sep

The hot, typhoon-prone months: boat and kayak trips are the first thing called off, and plans need a weather eye.

Year-round The pale columns of volcanic rhyolite, the boardwalk sea cave and the East Dam geology trail are here every day, and the sky above the reservoir is Hong Kong's darkest each night. What shifts is the weather, the sea state and the Milky Way.

The Sai Kung columns rise a hundred metres out of the South China Sea at the eastern edge of Hong Kong, and they are the wrong colour. Almost every famous colonnade in the world is dark basalt; these are pale rhyolite, full of silica. A supervolcano emptied itself here about 140 million years ago, and the ash that filled its caldera fused and cooled so evenly that it cracked into six-sided prisms. What makes these ones unusual is not the shape but the size. The slower a sheet of rock sheds its heat, the further apart the cracks travel, so width is a record of cooling. These average 1.2 metres across and the broadest reach three, which points to an ash sheet up to 400 metres thick that held its warmth a very long time. The flat path along the East Dam is the only stretch you can reach without a boat. Up close the hexagon tops are packed together like paving, their edges rounded off by salt spray and rain. This is the one place you can put a hand on rock 140 million years old. A boardwalk drops from it to the mouth of a sea cave. At the waterline the columns step down like a staircase cut in a hurry, and every wave threads the gaps between them under you. Seven thousand interlocking concrete blocks lie heaped against the ocean face of the dam, breaking up the swell. Boats and kayaks from Sai Kung town round the headland for the view no footpath gives. Po Pin Chau is a slab the sea cut off the cliff behind it: first a cave, then an arch, then a free-standing stack of columns dropping into deep water. After the last minibus the dam empties, nothing on it is lit, and the reservoir turns into one long black mirror. That reservoir was never dug. Two dams sealed a sea channel at either end, the seawater was pumped out, and the emptied strait was filled with fresh water. Cutting the East Dam is what opened the rock face you have been walking along.

Worth knowing

Good to know

Private cars need a permit to drive in — that restriction keeps the crowds thin, and the only length of columns you can reach on foot is the geo trail the reservoir dam sliced open.
Boat and kayak trips are called off on choppy days — the sea-level view of Po Pin Chau depends entirely on the swell, so keep a spare day if those cliffs are the whole point.
Stay off the dolosse — the concrete blocks are the seawall's working face, swell washes straight over them, and the geopark bans climbing on them and on the columns.
Carry on past the dam to the Po Pin Chau platform — a short climb beyond the geo trail opens the channel where the sea stack was severed from the headland.

Things to do

1
Walk the High Island Geo Trail
The flat path along the East Dam is the only part of the geopark you can reach without a boat. It runs past the rock face that dam builders sliced open in the 1970s, past the wall of S-bent columns, and down a boardwalk to the mouth of a sea cave. Up close the hexagon tops are packed together like paving, each about 1.2 metres across, their edges rounded off by salt spray and rain. This is the one place you can lay a hand on rock 140 million years old.
2
See the concrete blocks armouring the East Dam
Some 7,000 interlocking concrete blocks, a type engineers call dolosse, are heaped against the dam's ocean face. They break up the South China Sea swell so the freshwater reservoir behind the dam stays calm. The engineering sits directly against the volcanic rock it was built to shield.
3
Boat beneath the Po Pin Chau sea cliffs
Chartered boats and kayaks from Sai Kung town round the headland for a view no footpath can match. Po Pin Chau is a slab the sea carved off the cliff behind it — first a cave, then an arch, then a free-standing stack of columns dropping into deep water.
4
Watch the swell work the hexagon shoreline
At the waterline the columns step down like a staircase cut by someone in a hurry, and every wave threads through the gaps between them. Hong Kong is not where most people expect to spend an afternoon looking at rock rather than skyline, and that is the whole point of coming out here.
5
Stay for the Milky Way over the columns
Almost everyone leaves on the last minibus, and the dam empties out. There is no lighting on it: the reservoir turns into one long black mirror, the columns go to silhouette, and on a clear summer night the Milky Way climbs over the sea in front of you.