<P> In the Pennine millstone grit areas above an altitude of 900 feet (270 m) the topsoil is so acidic, pH 2 to 4, that it can grow only bracken, heather, sphagnum, and coarse grasses such as cottongrass, purple moor grass and heath rush . </P> <P> As the Ice age glacial sheets retreated c. 11,500 BC trees returned and archaeological palynology can identify their species . The first trees to settle 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 . </P> <P> 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 7 ft (2 m) thick . The erosion of peat still exposes stumps of ancient trees . </P> <P> "In digging it away they frequently find vast fir trees, perfectly sound, and some oaks ..." </P>

How many rivers were formed from the river of eden