Check out the recently installed GeoRegion trail sign in Bicentennial Park, West Pymble.
The cliff face bordering the east side of Lofberg Oval is a former quarry face that once yielded a hard, recrystallised form of Hawkesbury Sandstone, actually known as quartzite. The sandstone was hardened by the intrusion of a north north-east aligned basalt dyke, now deeply weathered but still exposed, tucked into a notch in the old quarry face.
Grants Castle is located roughly 1.3 km south of Bicentennial Park and doesn’t appear to have any surviving igneous body adjacent to it.
See Ku-ring-gai GeoRegion website for more details.