The rocks that came down
When the shakes opened the Port Hills, the cliffs threw their stones, and surveyors walked the slopes afterwards and mapped every one. 6,795 pins, 20,284 rocks in all: 12,042 came down, and 7,101 of the ones still sitting wobble under a push, the next shake’s ammunition.
The hills, surveyed
Every pin over the suburb outlines: red ticks are rocks the survey marked fallen, amber are the hanging ones a surveyor could move, grey are the rest. The band is the Port Hills themselves.
The biggest on record
By shape
What holds them up
The survey’s own read of what keeps each rock in place, a pin can name more than one hold, so the counts overlap. 951 rocks still rest on a tree: roots, not rock, holding them where they sit.
The hills' suburbs
By the rocks counted under each suburb, a pin can stand for a cluster.
- Sumner, 6,354 rocks counted, 1,176 wobble easy · 3,792 down already
- Heathcote Valley, 5,632 rocks counted, 279 wobble easy · 3,375 down already
- Lyttelton, 1,023 rocks counted, 48 wobble easy · 612 down already
- Mount Pleasant, 1,000 rocks counted, 34 wobble easy · 823 down already
- Hillsborough, 923 rocks counted, 73 wobble easy · 612 down already
- Governors Bay, 772 rocks counted, 139 wobble easy · 136 down already
- Cass Bay, 762 rocks counted, 71 wobble easy · 419 down already
- Cashmere, 510 rocks counted, 57 wobble easy · 212 down already
- Scarborough, 457 rocks counted, 31 wobble easy · 381 down already
- Richmond Hill, 408 rocks counted, 8 wobble easy · 400 down already
Each survey pin files under the suburb it sits in: the count of pins, the rocks counted (a pin may stand for a cluster), and how many rocks fell or wobble easy. A pin marks where the survey saw the rock, not where it is now.
Questions, answered briefly
After the shakes opened the Port Hills, surveyors walked the slopes and mapped the rocks, 6,795 pins, 20,284 rocks in all, since one pin can stand for a cluster. Each record carries the boulder’s size, its shape, and the survey’s own two verdicts: whether it came down, and whether it would move when pushed.
12,042 of them, the survey marked those "fallen". The rest sit where they always sat, but "insitu" isn’t "safe": the wobble test found 7,101 rocks a surveyor could move by hand. Those are the hills' unfinished business, the rockfall the next big shake hasn’t thrown yet.
The survey’s own field check, a surveyor puts a shoulder to the boulder. "Easy" means it moves under a push: the rock is barely held, the next shake’s candidate. "Difficult" holds but moves; "not tried" and unrecorded pins are what they are, seen, sized, not pushed.
Not necessarily, a pin marks where the survey saw it, not where it is now. Fallen rocks were cleared or left where they stopped; the hanging ones are a hazard note, not a promise. A point’s own answer, how far the nearest recorded boulder sits, is at /before.
The record behind this
Christchurch City Council’s open land layers, the BoulderFieldCapture layer of OpenData/LandCharacteristic, the GNS rockfall survey of the Port Hills. A pin is one record, and a record can stand for a cluster, "rocks" is the survey’s count at the pin. A pin marks where the rock was seen, not where it may be now: the fallen were cleared or left, the hanging are a hazard note, not a promise. Ask a point’s own record at /before; the breaks that threw them are at /sequence; the line that opened is at /faults; what the city bolted on after is at /defences.
This page as markdown: /boulders.md · the record as data: /boulders.json · /boulders.geojson · /suburb-boulders.json · on the live map: /map?layer=boulders.