The largest town in Craven is Skipton. Other major population centres in the region include High Bentham, Settle, Grassington. The expanded villages of Sutton-in-Craven, Cross Hills and Glusburn are now considered one urban conglomerate. Geography Craven comprises the upper reaches of Airedale, Wharfedale, Ribblesdale and the river Wenning of Lonsdale. Topography Craven is a group of valleys. Through Craven the River Aire and River Wharfe flow east to the North Sea; and the River Ribble and River Wenning flow west to the Irish Sea. To Craven's north stand limestone mountains of up to above mean sea level and to its south lie bleak sandstone moors, that above grow little but bracken. Transport can find the Pennines a formidable barrier for roads can be blocked by snow for several days. However, Craven makes a sheltered passageway with low passes. Natural vegetation At the end of the last ice age, ca 11,500 years ago, plants returned to the bare earth and archaeological palynology can identify their species. The first trees to colonize were willow, birch and juniper, followed later by alder and pine. By 6500 BC temperatures were warmer and woodlands covered 90% of the dales with mostly pine, elm, lime and oak. On the limestone soils the oak was slower to colonize and pine and birch predominated. Around 3000 BC a noticeable decline in tree pollen indicates that Neolithic farmers were clearing woodland to increase grazing for domestic livestock, and studies at Linton Mires and Eshton Tarn find an increase in grassland species in Craven. On poorly drained impermeable areas of millstone grit, shale or clays the topsoil gets waterlogged in Winter and Spring. Here tree suppression combined with the heavier rainfall results in blanket bog up to thick. The erosion of peat ca 2010 still exposes stumps of ancient trees. Vegetation in the Pennines is adapted to subarctic climates, but altitude and acidity are also factors. For example, on Sutton Moor the millstone grit's topsoil below has a soil ph that is almost neutral, ph 6 to 7, and so grows good grazing. However, above it is acidic, ph 2 to 4, and so can grow only bracken, heather, sphagnum, and coarse grasses such as cottongrass, purple moor grass and heath rush. However dressing it with lime produces better quality grass for sheep grazing. Such is named marginal upland grazing. This suggests that early pastoral farming on millstone grit soil