The spillway is located about away from the dam wall in mostly unweathered Ordovician andesite. The weathered and unweathered forms of the Weald Clay have different physical properties. On these hills, the unweathered limestone bed can be exposed by removing shallow overburden. It amounts to tens of kilo-Newtons in the case of unweathered quartzite. The base of the solum is the relatively unweathered parent material. The flow was unweathered, with little vegetation, indicating recent origin. Besides having been extensively fractured by frost action, it is unweathered. Unweathered rock is hard, dark greenish black and weathers to grayish olive.