 By the late 1860s copper mining in the area was declining, and by 1870 it became a depression, when the Clifford Amalgamated Mines closed.This resulted in a disastrous loss of income, and decline continued.Catastrophe struck in the heavy winter of 1876-77 when the Great County Adit (a common drainage system formerly used by several deep mines) which had fallen into disrepair and become blocked, suddenly burst and caused the instant silting of Devoran Harbour, forcing vessels to discharge by tender.