Palaeoecological data from Cronkley Fell in the Upper Teesdale National Nature Reserve emphasize the relict nature of the disjunct plant species which comprise the 'Teesdale assemblage'. The present distribution of these plants is however, recent, a result of complex ecological changes which have taken place during the Flandrian period, and is more extensive than at any time since late-Devensian times. Much of Cronkley Fell was covered by woodland and blanket peat in the early and mid-Flandrian period and the Teesdale assemblage was restricted to refugia, such as cliffs and screes, river alluvium and flushes. Some of these refugia were covered by blanket peat but, following the disappearance of the woodland around 3000 b.p., additional habitats became available for colonization by the relict plants. It is in the context of ecological change during the last Io ooo 000-12 ooo years that conservation of this unique flora must be viewed. QUATERNARY scientists have suggested that their value to modern society may lie in the historical perspective they can give to contemporary environmental problems (Washburn, 1970; Wright, 1973; Bryson, 1974). Recent palaeoecological studies in northern Minnesota, U.S.A. have provided striking illustration of this. Bradbury and Waddington (1973, P. 190) documented the biological and chemical changes in a lake since i88o and observed that 'The information and perspective provided by such historical analyses may ultimately make it possible to detect undesirable trends in lake pollution and to arrest them before unnecessary harm is done to these ecosystems.' Wright related ecological and geological theories of landscape development to argue for the continuance or return of those processes which characterize the Boundary Waters Canoe Area, part of the U.S. Wilderness Preservation System (Wright, 1974). This paper presents palaeoecological data documenting the Flandrian history and present status of the rare plants which today characterize the Upper Teesdale National Nature Reserve (Fig. i). Such data may be useful to those concerned with the use(s) to which the National Nature Reserve is put and may give more precision to arguments about the conservation of the unique flora. Pollen statistics presented here are from Cronkley Fell, Yorkshire, and are interpreted from the viewpoint of the survival of a group of rare plants or the existence of environmental conditions which tend to favour such restricted species in Upper Teesdale today. A summary of these data was published previously (Squires, 1971). Pollen data from other portions of Upper Teesdale have been published elsewhere (Johnson and Dunham, 1963; Turner et al., 1973). CONSERVATION AND THE TIME SCALE Although rejected as unattainable by Hardin (1968) and Fife (i971), a common definition of