Devil’s Boots

The Devil’s Boots are a limestone structure formed by differential erosion.

The boots comprise Nile Group limestone which is between 24 and 34 million years old, and formed as marine sediment when much of New Zealand was beneath the surface of the ocean. These rocks were deposited in a shallow marine continental shelf environment, and comprise the remains of bivalves, gastropods, brachiopods, and bryozoans, along with a limited amount of sand and other terrestrial-derived sediments.

Over time, limestone naturally erodes as a result of dissolution, which occurs when the slightly alkaline limestone chemically reacts with rainwater and groundwater to form a weak carbonic acid which slowly dissolves the rock. Water flowing over the limestone follows cracks and weaknesses in the rock and chemically weathers them over time. Over many thousands of years, this process is enough to carve out river channels, sinkholes and caves.

In the past, this entire region would have been covered in limestone; nowadays, bands and pockets of less erodible limestone are all that remain. The preservation of the boots could be due to slight chemical differences in the rock making them less susceptible to dissolution, or something as simple as the course of localised rivers coincidentally avoiding the structures.

Try and picture the entire landscape around you covered in the same limestone that makes up the Devil’s Boots. Take a moment to consider how much erosion must be happening to remove all of that limestone!

The chemical weathering of limestone and similar rocks produces what geomorphologists call karst landforms. Karst weathering patterns typically feature fluting – ribbon-like near vertical grooves or channels on the surface of the rock where water preferentially runs. Stand back from the boots and see if you can observe any karst patterns.