Beyond the Jurassic ridge the landscape of Britain becomes more varied. Considerable expanses of old hard rock have created barren mountainous areas like the core ofWales and the Highlands of Scotland while the predominantly westerly air currents ensure that the Atlantic coasts have a higher precipitation rate than much of the rest of the country. Since height above sea-level (which affects temperature) and mean annual rainfall are crucial factors in controlling crop growth, considerable areas of the north and west lie at or beyond the limit of viable agriculture. Thus, slight fluctuations in climate can cause major dislocations in settlement pattern (pp. 57-8). This factor, and the harsh nature of some of the environments, has tended to create an intense regionalism in settlement form and a high degree of specialization in economy. In the discussion to follow each major region is treated separately.