Formation of a string and pool topography as expressed by morphology, stratigraphy and current processes on a mire in Kuusamo, Finland

A review is made of earlier work and theories on a formation of string mires, together with a report on detailed investigations carried out over seven years on a small string ( aapa ) mire in eastern Finland, to determine the applicability of earlier hypotheses on the explanation of string and pool patterning. Attention is paid to the topography of the surface and bottom of the mire and to the peat and pollen stratigraphy. Results are presented of year‐round temperature measurements and long sequences of snow and frost depth readings from both pools and strings. Snow depth is shown to be a decisive factor in ground frost formation. The strings are shown by a series of repeated Geodimeter measurements to move partly upslope, partly downslope and partly in a sideways direction. The maximum cumulative movement recorded was about 1 m in seven years. Some of the pools remained unfrozen even in midwinter because of groundwater flow. Uneven retention and discharge of the surface water causes the strings to be in a labile state and promotes their movement. The initial formation of the string and pool topography is dated to 2,000–3,000 radiocarbon years B.P. and attributed to a rise in the groundwater table due to climatic change. The resulting fluviodynamics of the mire surface led to the accumulation of loose material into ramparts at the spring flood season and led in time to differential peat formation conditions, progressive peat accumulation on the strings, and regressive peat degradation in the pools. Thus the strings became higher and the pools deeper. The authors believe the importance of frost action, ice expansion and solifluction on the development of string and pool patterns is frequently overestimated.