The North Rocks were a massive sandstone outcrop, which terminated the ridge on the south side of Hunts Creek. These rocks so dominated the landscape and were such prominent features that they gave their name to the locality but were not preserved. In 1841 when it was decided to build a new gaol at Parramatta, a contractor bought the Rocks as they consisted of the best sandstone in the district - and were very conveniently situated. His tender was accepted, and much of the North Rocks became gaol walls and flagstones in 1844. Further inroads were made on what was left when the wall of masonry was erected in 1855-56 which held back the waters of Lake Parramatta. Much stone was needed for the dam, as the wall is about long and is fifteen feet thick at the base.