Police said the woman who died was 91 and from Crowborough. She was travelling north in a grey Volkswagen Eos convertible driven by a 68-year-old woman from Rodmell, near Lewes.
The driver was taken to the Royal Sussex County Hospital, Brighton, and detained for treatment.
Two other vehicles travelling in the opposite direction – a black Ford Fiesta driven by an 80-year-old man from Uckfield and a silver Audi A3, driven by a 62-year-old woman from Rotherfield – took avoiding action. No one from these vehicles was hurt.
Officers want to speak to anyone who witnessed the collision, particularly the driver of a tanker which was travelling northbound through Heron’s Ghyll at the time.
The road was closed for more than five hours while an investigation took place and the scene was cleared.
The Kent, Surrey and Sussex Air Ambulance was called and landed nearby in a field. Its crew assisted at the scene.
Anyone who witnessed the collision or has any information is asked to contact police by emailing email@example.com, reporting details online here or by ringing 101, quoting Operation Chepstow.