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Calcareous fens in J iimtland193CALCAREOUS FENS IN JAMTLANDBy Yngve NordqvistThis section will deal with mires that to someextent at least are characterized by the occurrenceof strongly calciphilous plants (Du RIETZ 1949a,p. 303) and by a high content of Ca in their water(WITTING 1949, p. 738) and peat, as well as highpH values. The typical vegetation is that of the"extreme rich fen" or Euscorpidion (Du RIETZ,Le.), but less rich vegetation usually occurs aswell.Mires of this kind have a wide range in theCambro-Silurian area of central Jamtland, but theyalso occur elsewhere in this province, e.g. on thewestern, more or less metamorphic Cambro-Silurianfacies of the Scandes, and in that part of the Archaeanarea to the east which is partly covered bycalcareous drift carried eastward by ice during latestages of the last glacial period, when the icedivide had its westernmost position. For topographicalreasons, large parts of the Cambro-Silurianarea of central Jamtland have a very high percentageof mire, as stated in BJORKBACK's contribution.Only one sizable work has been published onthese mires, viz. BooBERG 's (1930) "Gisselasmyren",being a monograph on a single mire. Other worksare few and not comprehensive but deal with variousaspects of vegetation and ecology; the flora is fairlywell known (LANGE 1938).The description below refers to field work onArasmyren, like Gisselasmyren situated on theHammerdal plain, but still intact whereas the lattermire has long been ditched. However, the accounthas been supplemented to some extent with datafrom the literature and impressions from variousother mires visited, although no attempt has beenmade to describe the full range of variation. Thetypes of poor fen and ombrotrophic bog that occuron parts of the otherwise calcareous mires are thusleft out, and the moderately (transitional) rich fenshave not been treated comprehensively. The classificationis tentative only.HYDRO-TOPOGRAPHICAL CONDITIONS .-The majorityof the calcareous mires of Jamtland occur inthe weakly undulating or almost flat Cambro­Silurian area, alongside and in the bottoms ofbroad, flat valleys or on plains or occasionally onlow plateaux. Frequently mires extend on eachside of rivers or lakes (BooBERG, p. 266). Thereare usually many low morainic hills of variable sizeand shape dividing the mires into smaller parts orat least reducing the extent of open areas. Asshallow margins and parts adjacent to rivers andlake shores are frequently wooded, mainly by birch(Betula pubescens) and spruce (Picea abies), thereare only a few wide and far-reaching open vistasover large expanses despite the great areal extensionof mires.A typical mire in the Cambro-Silurian area slopesbut slightly (often about 1 in 50) and is dividedinto small or moderately large open fen expansesconnected by narrower straits known as "mirenecks" and conducting water in roughly parallelseepages of a soligenous character. Both Gisselasmyrenand Arasmyren belong to this type. Otherparts f the mires are wet and almost horizontal,i.e. topogenous or limnogenous (SJORS 1950b, p.208), chiefly localized in flat valley bottoms. Anexample with a complicated drainage, includingboth areas regularly flooded by a river and areaswhere superficial water moves along faint slopes,is Sikasva.gen between Hammerdal and Gaxsjo.Here an area with widely convergent water movement,known as Mar a vagsflon, has been studied;despite a very faint slope it shows regular developmentof low "strings" at right angle . to the slope.The extent to which such patterns have been developedis very variable on the Cambro-Silurian whichis climatically not among the most favourable areasfor pattern formation, having neither very cold"continental" winters nor the excessive precipitationof the west.Strongly sloping £ens are characteristic of theActa Phytogeogr. Sue c. 50