Refers to the area where the wide portion of Capertee Valley and River narrows between two headlands. Descriptive of the valley and river narrowing into a mouth. Sir Johns Point and Mount Gundangaroo form the southern side and Mount Iris is on the northern side. It is also the area where the present Glen Alice Road from the north and Glen Davis Road from the west intersect. Named after Sir John Jamison (1776-1844), physician, landowner and constitutional reformer. Shown on Map of the Colony of New South Wales by Thomas Mitchell, 1834. "Sir Johns Mouth, the river has eroded its channel though both the Hawkesbury Series and the Coal measures, and for some depth into the Upper Marine strata, leaving on either hand bold headlands capped with sandstone cliffs." Ref: Carne, Joseph. The Kerosene Shale Deposits of New South Wales, 1903, p. 166.