Location
Royal National Park, NSW
Rock
Hawkesbury Sandstone
Age
Middle Triassic · ~240 Ma
Process
Honeycomb (alveolar) weathering
Status
Fenced · collapse risk since 2015
Overview
Where tafoni are rooms, honeycomb weathering is lace: tight fields of small cavities split by thin walls. On the Royal National Park coast it frames one of Sydney's most photographed and most fragile rocks.
25m
Height of Wedding Cake Rock above the sea
2015
Year NPWS fenced the rock off
10yrs
Window in which a 2015 survey said it could collapse
Chapter 01
Honeycomb weathering, also called alveoli, stonelace or fretting, is a form of cavernous weathering: regular, tightly adjoining cavities that develop where salt and moisture work at a fine scale, leaving narrow ridges between them.
Chapter 02
Wedding Cake Rock is a bright, layered block of Hawkesbury Sandstone. Its colour comes from iron being leached out of the sandstone, which also leaves the layers soft and prone to cracking.
Chapter 03
In May 2015 NSW National Parks fenced off the rock. A June 2015 geotechnical survey found it severely undercut and precariously balanced on the cliff edge, and concluded it could collapse at any time within ten years.
Chapter 04
Nobody sets the size of the cells. Ridges dry quickly and resist; pits stay damp and keep weathering. The balance between the two settles into a spacing that repeats across the face.
Chapter 05
The same softening that makes the lace so fine has undercut Wedding Cake Rock. In 2015 a geotechnical survey found it precariously balanced, and the headland has been fenced off since.
How it forms
Spray and rain wet a flat face of sandstone. Salt enters every pore.
Tiny pits appear where the rock is slightly weaker or stays wet slightly longer.
Neighbouring pits compete for moisture. The ridges between them dry fastest and survive.
Walls thin to a regular network: honeycomb weathering, also called alveoli or stonelace.
A cliff that looks like sugar icing, and is nearly as soft.
The question
Each pit's growth is limited by its neighbours: it can only expand until it meets another pit's wall. Over time that local rule produces a near-regular pattern with no designer, the way foam or cracked mud does.
Honeycomb (alveolar) weathering
Deep time
~240 Ma
Sandstone
The Hawkesbury Sandstone forms from Triassic river sands.
~7,000 yrs
Salt coast
The modern coastline stabilises and salt spray reaches the cliff tops.
May 2015
Fenced
NSW National Parks closes access to Wedding Cake Rock.
June 2015
Undercut
A geotechnical survey finds the rock severely undercut and at risk of collapse.
Field record
Field data
IGMA simulation · move to heat
Rock Lab
Turn a 3D model of Honeycomb Lace with hand gestures, heat its surface with your fingertip and weather it through 250,000 years.
Enter the Rock LabReferences
Photography
Next case · 03
A granite breaker, frozen mid-curl