I took my first walk of 2013 around our well-trodden loop in Upper Teesdale on Saturday, parking near Harwood Beck, walking along the track to Widdy Bank Farm, then following the course of the Pennine Way as far as Cauldron Snout before skirting the edge of Cow Green Reservoir to pick up the minor road back down the hill to the car. It is about 17 kilometres in total and I probably walk it three or four times each year, watching how the landscape changes with the seasons.
Upper Teesdale in February. The picture on the left shows Widdy Bank Farm on the right hand side; the picture on the right is a view looking downstream from a point near Cauldron Snout. The cliffs and screes of Falcon Clints are visible on the left hand side of the image.
The section from Widdy Bank Farm to the top of Cauldron Snout is the most dramatic part of the route, as the river funnels through the gap between Falcon Clints and Cronkley Scar. Landscapes in this area are more dramatic than elsewhere in the northern Pennines because the underlying rock is the hard Whin Sill, a dolerite formed by volcanic activity at the end of the Carboniferous period almost 300 million years ago.
It is not just the landscape that is different here. Scrambling along the scree at the base of the cliff, I see the distinctive yellow-green patches of the map lichen Rhizocarpon geographicum. The name comes from the appearance as the individual patches, each scattered with black fruiting bodies, merge into a patchwork. This lichen is favoured both by the clean air of upper Teesdale but also by the base-poor substratum that the dolerite provides.
Rhizocarpon geographicum growing on the dolerite scree below Falcon Clints in upper Teesdale.
Further along, and scrambling over yet more scree, I saw patches of a greyish-green moss on top of the dolerite boulders. Closer examination shows that the leaves all have a narrow lanceolate shape, gradually tapering to a long, colourless hair. This moss is Racomitrium heterostichum. The genus is quite distinctive even with the naked eye but, under the microscope, the characteristically sinuous leaf cells with their thick walls are another useful diagnostic characteristic. Seen this way, the leaves are as green as those of any other plant; however, our view is normally filtered through the tangle of hairs, leading to the distinctive appearance.
The left hand image shows Racomitrium heterostichum growing on a dolerite boulder in Upper Teesdale; the right hand image shows leaf cells at high magnification. Each is approximately 1/100th of a millimetre wide.
Though I was standing in the usually moist environment of the northern Pennines, surrounded by snow-covered hills, I was actually looking at a miniature desert. The upper surface of a boulder is a harsh and inhospitable environment for any organism. When it rains, the moss can photosynthesise, even at relatively low temperatures. However, for much of the year the moss will be too dry to grow. The long hairs create a white reflective layer over the top of the cells, which cools them down and, perhaps, slowing evaporation of water. The hairs will also help to trap mineral particles from which the moss will gain nutrition.
Had I looked harder, I would probably have found many more organisms hiding away amidst the stems of Racomitrium, taking advantage of the shelter and milder microclimate that they offer compared to the face of the boulder itself. To push the metaphor from the previous paragraph just a little further, these patches of moss are tiny oases amidst the savage desert conditions that the boulders present.
Follow this link for a useful review of the ecology of mosses