Coffee rock

Coffee rock is the common name for the rock-like formations of indurated sands that were formed from ancient river sediments of the Pleistocene age. It is characterised by being a soft, dark-brown friable rock which had been cemented together by organic matter (mostly plant based). It is formed when humic substances are washed through sandy soils where they meet shallow aluminum-rich groundwater.^[1]

Coffee rock on K'Gari.

Coffee rock has been exposed by coastal weathering process on the beaches of Broadwater and Bundjalung National Parks in New South Wales, Australia.^[2] In addition, exposures can be seen in North Queensland, Australia at Kurrimine Beach, and in South East Queensland on K'gari and at the entry to Coonowrin Lake in Caloundra.

Exposed Coffee Rock on the beaches Fraser Island is more likely the beds of old lakes in the sand dunes when the sea level was lower and one of the most well known sites of it on the island is at Bowarrady Creek.^[3] In places it is peat-like and embedded with wood ranging from small twigs to large tree trunks up to 1200mm in diameter, with some evidence of fire on the wood before being assembled.^[1][4]