Red and white sandstones are exposed in a gully in the northern part of Ben Boyd National Park and are known as the Pinnacles. These sandstones are eroded remnants of a much larger extent of sandstones of Neogene (Tertiary) age, about 35 million years old. The sandstones were deposited in creeks and rivers in a coastal hinterland.
The sandstones have remained soft and easily eroded, allowing a small stream to cut through them exposing this slice of geological history. Erosion continues today, forming the small gullies that give the outcrop its name, the Pinnacles.
The rusty red colour of the sandstones at the Pinnacles is due to iron oxides which provide a clue to the climate at that time: warm, with high rainfall, interspersed with arid periods. Vegetation would have been warm temperate to tropical rainforest.
The boundary between the red and the white sandstones demonstrates the former presence of groundwater that permeated the sandstones long after they were originally deposited and weathered. Groundwaters often contain plant acids or other natural chemicals that can bleach the colour from iron oxides. The line between the white and the red-coloured sandstones shows where the top of the water table sat.