A long cold climatic spell prevailed until the end of the last glacial period about 9,000 years ago, and most of Ireland was covered with ice. Sea-levels were lower then, and Ireland, as with its neighbour Britain, rather than being islands, were part of a greater continental Europe. Mesolithic stone age inhabitants arrived some time after 8000 BC. Agriculture arrived with the Neolithic circa 4500 to 4000 BC, when sheep, goats, cattle and cereals were imported from southwest continental Europe. At the Céide Fields in County Mayo, an extensive Neolithic field system – arguably the oldest in the world – has been preserved beneath a blanket of peat. Consisting of small fields separated from one another by dry-stone walls, the Céide Fields were farmed for several centuries between 3500 and 3000 BC. Wheat and barley were the principal crops.