Ashfield Shale is part of the Wianamatta group of sedimentary rocks in the Sydney Basin.

[1] At Turramurra 33 metres remains and formerly there was a good deep exposure of it at the Railway Station until this suffered shotcreting.

The fine grained silty sediments were laid down in a low energy, south-east flowing deltaic setting, near the shores of a shallow sea.

Weathering of the shale units produces a reddish/brown podsolic soil, often with poor drainage, such as that in the Cumberland Plain.

The adjacent Hawkesbury Sandstone is considered a safer bedrock than the (less stable and laminated) Ashfield Shale for building construction.

In 2005, the construction of the Lane Cove Tunnel was affected by the collapse of an exit ramp excavation, through Ashfield Shale.

[4] Difficulties may be encountered where the Ashfield Shale interfaces with the Hawkesbury Sandstone and the Mittagong Formation.

It is known from a skull roof impression found in the Ashfield Shale at the old Hurstville Brick Company quarry at Mortdale.