The history of agriculture in Scotland includes all forms of farm production in the modern boundaries of Scotland, from the prehistoric era to the present day. Scotland's good arable and pastoral land is found mostly in the south and east of the country. Heavy rainfall, wind and salt spray, in combination with thin soil and overgrazing, made most of the western islands treeless. The terrain often made internal land communication difficult, encouraging a coastal network. In the Neolithic period, from around 6,000 years ago, there is evidence of permanent settlements and farming. The two main sources of food were grain and cow milk. From the Bronze Age, arable land spread at the expense of forest. From the Iron Age, there were hill forts in southern Scotland associated with cultivation ridge