Whitecliffs Boulders

These concretionary boulders have eroded out of a localised layer of sandstone within the surrounding papa (mudstone) – the relatively soft light blue-grey mudstone rock that forms the surrounding cliffs and underlying bedrock. The concretions are made of fine to medium sandstone, which is a coarser material than the mudstone that forms the bulk of the cliffs.

Microscopic fossils called foraminifera are abundant in the bedrock and they suggest that the mudstone that surrounds this site was deposited on the seafloor about 3 to 2 million-years ago, in water depths of about 50-150 m.
As layers of calcareous muddy sediment slowly accumulated on the seafloor at a rate of about 1m per 1000 years, the sea floor itself was subsiding. Slowly the material became buried to depths of several thousand metres.

Percolating deep groundwater (sourced from rain above) flowed through the more permeable sandy layers and dissolved calcium carbonate (lime) from the tiny fossil shells in the surrounding rocks. The groundwaters became saturated in the calcium carbonate and this began to crystallise out when it encountered something different in the sand. These were small blocks of mudstone (10-40 cm in diameter) that became the nuclei for the calcite crystals that grew outwards in all directions around them. The crystals grew in the voids between the sand grains and cementing the grains together gradually creating the hard, erosion-resistant spherical boulder concretions. In this way the hard cemented concretion would have slowly grown outwards (within the sandstone layer). The large size of many of the Whitecliffs concretions suggests that this process may have taken tens of thousands of years.

The concretions are much more durable than the surrounding mudstone, so once the subsidence stopped and the rocks were uplifted to their present level (about 300m above sea level), the mudstone was more easily eroded away, leaving the concretions to accumulate on the surface. Their exact sandstone source, and why they are all gathered into a very small area on the valley floor is something of a mystery.

Enjoy your exploration of the boulder forest. During your walk, see if you can find: 1) concretionary boulders with parallel layers of sandstone bedding; 2) the cores in split concretions and what they are made of.
Estimate the diameter of the largest concretionary boulder that you can find, and ask yourself, if the concretion grew at an average rate of 1-millimetre per decade, how many years did it take for the concretion to grow to that size. Then estimate how old you think the oldest tree is the forest, and ask yourself, what takes the longest time to grow – concretions or trees?
Why do you think so many of the concretions split in half so cleanly?
The concretions come from a sandstone layer or layers within the young mudstone rocks. Why do you think so many of the concretions are concentrated in this relatively small area beside the river. Did they: 1) roll down from the surrounding cliffs; 2) get washed into place by the river when it flowed at this level; or 3) are they more-or-less sitting where they formed?