Postglacial peatlands of the Lake Agassiz plain, northern Minnesota.

The eastern arm of Lake Agassiz is now largely covered by a vast peatland. The largest segment, north of Upper Red Lake, is 100km long and 20km wide. The western part consists of a sedge fen 5km wide, marked by teardrop-shaped islands of spruce Picea and dwarf birch Betula nana and by a pattern of linear ridges and pools transverse to the direction of water flow. The rest of the Red Lake peatland consists of complexes of nutrient-poor raised bogs and intervening water tracks; drainage off the higher raised bogs, converting them to streamlined ovoid islands 1-2km wide. The vegetation patterns are closely adjusted to water flow and water chemistry, and not to the mineral substratum. The pH increases from <4 at the crest of the raised bogs to c4.5 in the internal water tracks, perhaps because of ion release from decomposing peats, or perhaps because of discharge of groundwater on the flanks of the raised bogs. Radiocarbon dates show that the peatland has developed during the last 4000 yr, after the mid-Holocene prairie period. The peatland is apparently spreading westward in recent time, as are the conifer forests on the upland.-from Authors