Most of Ireland was covered with ice until the end of the last ice age over 9,000 years ago. Sea-levels were lower and Ireland, as with its neighbour Great Britain, were a part of continental Europe rather than being islands. Mesolithic stone age inhabitants arrived some time after 8,000 BC and agriculture followed with the Neolithic Age around 4,500 to 4,000 BC when sheep, goats, cattle and cereals were imported from the Iberian peninsula. At the Céide Fields, preserved beneath a blanket of peat in present-day County Mayo, is an extensive field system, arguably the oldest in the world, dating from not long after this period. Consisting of small divisions separated by dry-stone walls, the fields were farmed for several centuries between 3,500 and 3,000 BC. Wheat and barley were the principal crops. The Bronze Age – defined by the use of metal – began around 2,500 BC with technology changing people's everyday lives during this period through innovations such as the wheel, harnessing oxen, weaving textiles, brewing alcohol, and skillful metalworking, producing new weapons and tools, and fine gold decoration and jewellery, such as brooches and torcs. According to John T. Koch and others, Ireland in the Late Bronze Age was part of a maritime trading-networked culture called the Atlantic Bronze Age that also included Britain, France, Spain and Portugal where Celtic languages developed.