of the cultural or natural history of New South Wales. The cyclopean masonry of the dam is an excellent and early example of gravity dam construction in the Inter War era incorporating inspection galleries, contraction, joints and foundation drainage system which collectively demonstrate the principal characteristics of the state development of this technology at the time. The double level discharge, penstock gates and roller gates collectively demonstrate the principal characteristics of the state development of this technology at the time. The terraces and platforms adjoining the dam abutments demarcate the location of plant and equipment used to in the construction of the dam, in particular the location of the cableway head towers, the quarry railway terrace, the motor vehicle garage, and the electricity substation. The grounds of the dam retain numerous tree plantings undertaken from the time of the completion of the dam in 1928. Collectively the diversity of these trees present a good record of past horticultural practices. The catchment area in being relatively untouched bushland in close proximity to a major urban area has a high potential for further research into natural ecosystems. The place possesses uncommon, rare or endangered aspects of the cultural or natural history of New South Wales. The basin of the reservoir of Avon Dam is the area of the largest impoundment within the Upper Nepean Catchment Area. Avon Dam is one of three dams in NSW which incorporate extensive Inter War Egyptian Architectural detailing. Avon Dam is however unique in always retaining remnant landscape features that continues to evoke the imagery of an Egyptian revival landscape. Avon Dam is one of two dams in NSW which incorporate pedestrian and vehicular entry pavilions to the crest wall. The crest and valve houses and inlet works retain original ironwork and machinery such as the roller gates and penstock gates and operating mechanism which represent a substantial repository of water supply delivery technology of the era. The spillway channel was the largest in terms of the depth and width constructed up to that date within the Sydney metropolitan area. The purpose-built road of access to the dam wall from the railhead at Bargo is unique within the context of the four metropolitan Dams in being the principal means by which the general supplies, men and raw materials were transported during the construction process. The dam wall retains evidence for a scour outlet operating system which was