Omarama Clay Cliffs

Badlands style erosion is formed by wind and water erosion into soft rock (not well held together and usually clay-rich). It requires steep slopes with little or no vegetation or soil and a high density of drainage. It's not certain how long this process takes, and it may vary depending on the properties of the rock layers but it's thought that at this location the erosion has been active over at least the last 2 million years.

The Clay Cliffs expose sediments that represent old alluvial braided river and fan deposits derived from the uplifting of nearby mountain range fronts.The sediments have also been uplifted by the Ostler Fault Zone and the beds are now quite steeply inclined.

These sediments are composed of quartz rich siltstones, sandstones, and conglomerates. They belong to what is known as the 'Hawkdun Group' with two different sub-units exposed here: The pale grey material of the Wedderburn Formation near the base has more quartz and less lithic (mixed rock source) gravel than the darker material of the Maniototo Conglomerate overlying it. The Maniototo Conglomerate is composed of well rounded clasts/fragments of Rakaia terrane sandstone with some schist and is often weathered quite rusty red-brown.

Can you see the characteristic steeply inclined bedding exposed in the Clay Cliffs due to these sediments having been uplifted and tilted along the Ostler Fault Zone over time? The Ostler Fault Zone itself is located right along the front of the Clay Cliffs, at the base of the hill. It is an active fault zone with a slip rate of around 1-2 mm per year. From the Clay Cliffs, the fault zone extends to the north east.

Take a close look at the conglomerates. Look for rock clasts (pebbles, cobbles etc) stacked up on top of each other and leaning at an angle (like books on a shelf). This is called imbrication - the rocks are leaning in the direction of water flow. So can you work out the direction the current was flowing from? Alluvial fans will have changing imbrication directions because material is dumped in a conical fan shape out from the mouth of a gully at the range front. Therefore the water flows away in different directions down the fan.