The geological story of Matakohe Limestone Island is one that begun approximately 25 to 37 million years ago when the giant land mass known as Zealandia, became submerged beneath the sea and almost disappeared. The rocks that form Matakohe Limestone Island were part of a vast wedge of rocks that were formed offshore to the northwest of present day Northland. Through the forces of massive tectonic activity they were pushed in a south westerly direction about 24-21 million years ago. During this emplacement the rocks were fractured, tilted, folded, torn apart and stacked on top of one another. The evidence of this dramatic action can be found in the rock forms of Matakohe Limestone Island today.
The rocks found on Matakohe Limestone Island are all of a type known as sedimentary rocks, which as the name suggests, were formed through the settling of loosened particles in the deepening sea basin during the geological period named the Oligocene drowning. You can find three different types of these sea born sedimentary rocks on Matakohe Limestone Island; mudstone, sandstone and limestone. They formed in sequence from mudstone (oldest) through sandstone to limestone (youngest).
At low tide the dark grey mudstone can be seen on the shore platform on the northwestern side of the island. It is made up of very fine-cemented grains of rocks and contains microscopic fossils, which indicate the sediment forming this mudstone was deposited in a shallow marine setting approximately 34 to 37 million years ago.
Look out for small outcrops of sandstone with their distinctive green tinge from the mineral glauconite. These outcrops can be found on the southern and northern coasts of the island and inland near the pa site. The sandstone is more coarse in texture than the mudstone and was formed from sand sized grains of rocks being cemented together about 32-34 million years ago.
The bulk of Matakohe Limestone Island is limestone, which European settlers quarried to make cement and gave the island its English name. The limestone on the island is mixed with silt so it is known as muddy limestone and was formed approximately 34 to 37 million years ago. The limestone is mostly made of calcium carbonate from a deep-sea ooze consisting of the accumulated skeletal remains of marine organisms that settled on the sea floor. All of the shelled fossils in the limestone are from microscopic simple single celled organisms and invisible to the naked eye, but you may be able to find the large fossilised feeding traces of large worm like creatures. These can sometimes be seen in the coastal cliffs and in fallen rocks around the island.
When visiting Matakohe Limestone Island the ancient and dramatic story of its geological formation may initially not be so obvious. But once the traces of its tumultuous beginnings are explored, it will definitely add yet an extra dimension to this not so ordinary little island.