Other names for bogs include mire, quagmire, and muskeg; alkaline mires are called fens. It is unique to natural areas called peatlands, bogs, mires, moors, or muskegs. In areas where water accumulates, dangerous bogs or mires can result. The Endla nature reserve protects a fresh-water system of mires, bogs, springs and rivulets. Of these, 42% are raised bogs; 49% are fens; and 9% are transitional mires. These clay sediments are locally overlain by peat deposits forming raised mires. On these poor, infertile soils have developed heathland, valley mires and damp woodland. Lowland raised mires occur mainly in northern England and extend south to the Midlands. Very large swathes of Canada, northern Europe and northern Russia are covered by boreal mires. The peat in mires contain a substantial amount of organic matter, where humic acid dominates. The distribution of mires therefore depends on topography, climate, parent material, biota and time. Topographically, mires elevate the ground surface above the original topography. Tropical mires largely form where high precipitation is combined with poor conditions for drainage. In addition, most mires are generally net emitters of methane and nitrous oxide. The grass snake ("Natrix natrix") prefers the damper environment of the valley mires.