It is a novel book. During the last glacial period, and up until about 9000 years ago, most of Ireland was covered with ice, most of the time. Sea levels were lower and Ireland, like Great Britain, formed part of continental Europe. By 12,000 BC, rising sea levels due to ice melting caused Ireland to become separated from Great Britain. Later, around 5600 BC, Great Britain itself became separated from continental Europe. There is no evidence of any humans being in Ireland before Mesolithic people arrived by boat from Britain between 8000 BC and 7000 BC. From about 4500 BC Neolithic settlers arrived introducing cereal cultivars, a housing culture (similar to those of the same period in Scotland) and stone monuments. A more advanced agriculture was to develop. 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 3500 BC and 3000 BC. Wheat and barley were the principal crops imported from the Iberian Peninsula.