Wharariki Beach / Archway Islands

The rocky arches and stacks of Wharariki Beach are formed of Farewell Formation (lower Kapuni Group), around 55-65 million years old. The rocks are very distinctive quartz and feldspar rich sandstones and pebbly (gravel) conglomerates. Much of this region was above sea level during a relatively quiet period of tectonic activity. Significant erosion led to stripping of land-based sediment right up until the Middle Eocene (around 38 million years ago) to the Late Oligocene, when the land was finally submerged by tectonic subsidence.These rocks are therefore interpreted to have formed as the large quantities of sediments were pushed across an ancient coastal plain environment, either in a braided river floodplain, or high sediment meandering river system.

The process of coastal erosion is caused mainly by wave action in the surf zone, along with tides and currents. As waves are mostly an effect of wind, it is the top five or ten metres of the sea that is the most turbulent. This means that most coastal erosion occurs at or just below water level. The waves rush into the cliffs, undercutting them and giving rise to a great variety of features such as caves, blowholes, arches and stacks (of which you can see all but the blowholes at this location). Hard, resistant rocks like these sandstones and conglomerates form steep sea cliffs which stand out relative to bays where less durable material has been eroded away. Wharariki Beach is also known for it's almost constant wind! This is responsible for forming the extensive aeolian (wind formed) sand dunes between the cliffs, arches and stacks.

Wharariki Beach is a great place to explore with many interesting erosion features and easily accessible rock so you can get up close to look at the features.

Can you identify the different layers of sandstone versus conglomerate in the cliffs? Look carefully at the large clasts (bits) in the conglomerate - what type of rock are they made of? These bits of rocks represent material from pre-existing rock that was eroded out of the landscape upstream, transported downriver, then deposited near the coast on the river flood plain in the great deposits of conglomerate rock we see here now. The ages and types of these individual pieces of rock might actually vary greatly, depending on where they came from! When we give an age for the overall rock here (the conglomerate), we give the age when all the bits were deposited, not the ages of the bits within it!

The arrangement of the clasts in the rock can give you clues about which way the river water was flowing when those bits were deposited. This is known as imbrication. To work out the flow/current direction, look at the way the clasts in the rock are stacked up against each other like upright books sloping on a bookshelf. Is there a consistant way the clasts are stacked? If so, the current direction can be found in the same direction as the long edge of the clasts are pointing up towards, just like if you pushed over the pile of books!