The Cairngorms consists of high plateaux at about 1000–1200 m above sea level, above which domed summits (the eroded stumps of once much higher mountains ) rise to around 1300 m. Many of the summits have tors, free-standing rock outcrops that stand on top of the boulder-strewn landscape. In places, the edges of the plateaux form steep cliffs of granite and they are excellent for skiing, rock climbing and ice climbing. The Cairngorms form an arctic-alpine mountain environment, with tundra-like characteristics and long-lasting snow patches. This area is home to bird species such as ptarmigan, dotterel, snow bunting, curlew and red grouse, as well as mammals such as mountain hare. The plateaux also support Britain's only herd of reindeer. Surrounding the central massif are many remnants of the Caledonian forest in straths and glens of the Rivers Spey and Dee. These forests support many species that are rare elsewhere in Britain, including red squirrels, pine marten, wood ants, Scottish crossbill, capercaillie and crested tit.