Pancake Rocks

The Pancake Rocks are part of the Oligocene aged (around 22-30 million years old) Nile Group. The Nile Group limestones are present along much of the Northwest Nelson region and upper West Coast, consisting of two distinct rock groupings representing different environments. They formed on low-lying land that was drowned and submerged due to regional tectonic extension and subsidence. During this time, sediment supply from land was limited, leading to the formation of limestone and calcium carbonate rich deposits in the calm shallow seas.This location contains part of what is known as the Platform facies (usually <100 m thick), consisting of shallow-water limestone rich in broken shells and algal balls, and muddy limestone, representing stable continental shelf. While originally the limestone and other Oligocene sediments would have most likely covered the entire region, erosion, uplift and some burial over the last 22 million years has restricted the limestone to reasonably well defined bands.

The Pancake Rocks limestone has been eroded into spectacular landforms by a combination of coastal erosion (including collapse of some underground caverns) and karst erosion (slow chemical solution along joints and caverns) beneath relatively recently deposited marine gravels.

The distinctive 'pancakes' that Pancake Rocks are famous for are often referred to as 'stylobedding'. Although the origin of stylobedding is often debated, it is mostly agreed that this layering is not original bedding (which would normally be caused by deposition of different grain sizes). Instead, the pressure and compaction from overlying sediments has created thin alternating strong and weaker bands in the limestone. Limestone is made primarily of calcium carbonate, but can contain imperfections. These imperfections in the rock and crystal lattice can migrate under the pressure through a process called grain boundary diffusion. At this large scale, the imperfections align in a plane, making it weaker. Since the weaker bands are made up of fine, weakly-bonded clay minerals, it is much less durable, so these layers erode faster than the stronger, more intact, limestone, forming the ‘pancakes’.

Look carefully at some of the stylobedding in the limestone at the edges of the track. Can you see details in the different layers? What does the weaker, less durable material look like compared to the more intact limestone? Can you identify any small fossils visible in the limestone? Algal balls (which are quite common in this group) look like small cream blobs only a few millimetres across, sometimes longish and curvy in shape, like miniature Twisties!
The coastal erosion has carved some interesting shapes into the rock so a fun activity for the kids is seeing what sort of animals and figures you can spot!

The blowholes are also a highlight of this location, however, you usually need high tide and a big swell coming from the west to see them working! Blowholes are a special type of sea cave eroded primarily by coastal erosion. Weaknesses, such as joints or fault lines, are widened, primarily through physical wave action, but also through other processes such as chemical solution in limestone. As the underground sea cave grows landwards and upwards into a vertical shaft along the joints or faults, they might erode enough to expose themselves to the land surface. Successive waves trap air into the fissure or sea cave and compress it as the crest of the wave fills the space, resulting in pneumatic pressure. This is then released in upward blasts of water in a spectacular fashion from the top of the blowhole if the geometry of the cave, blowhole and water conditions (weather-formed waves and tides) are appropriate.