The defining factor in the geography of Scotland is the distinction between the Highlands and Islands in the north and west and the Lowlands in the south and east. The highlands are further divided into the Northwest Highlands and the Grampian Mountains by the fault line of the Great Glen. The Lowlands are divided into the fertile belt of the Central Lowlands and the higher terrain of the Southern Uplands, which included the Cheviot hills, over which the border with England ran. The Central Lowland belt averages about 50 miles in width and, because it contains most of the good quality agricultural land and has easier communications, could support most of the urbanisation and elements of conventional government. However, the Southern Uplands, and particularly the Highlands were economically less productive and much more difficult to govern. The Uplands and Highlands had a relatively short growing season, in the extreme case of the upper Grampians an ice free season of four months or less and for much of the Highlands and Uplands of seven months or less. The early modern period also saw the impact of the Little Ice Age, with 1564 seeing thirty-three days of continual frost, where rivers and lochs froze, leading to a series of subsistence crisis until the 1690s.