. The Canadian field-naturalist. 228 THE Canadian Field-Naturalist Vol. 98 the cave system. The water level of Horse Lake was controlled by the sinkholes. Remains of a beaver dam occur above the largest sinkhole at the edge of the present lake (Figure 1). This sinkhole is currently about m deep and up to 20 m wide. The placement of the dam around the edge of this sinkhole, closing- off a small channel which drains into the sink indicates that draining of the lake through these sinks predates the dam. It is not known if Beavers dammed the bay as it receded toward the centre of the lake. The
. The Canadian field-naturalist. 228 THE Canadian Field-Naturalist Vol. 98 the cave system. The water level of Horse Lake was controlled by the sinkholes. Remains of a beaver dam occur above the largest sinkhole at the edge of the present lake (Figure 1). This sinkhole is currently about m deep and up to 20 m wide. The placement of the dam around the edge of this sinkhole, closing- off a small channel which drains into the sink indicates that draining of the lake through these sinks predates the dam. It is not known if Beavers dammed the bay as it receded toward the centre of the lake. The dam stopped drainage into the large sinkhole and probably raised the level of the lake above current late summer levels. Continued subsurface drainage around the dam, however, eventually forced the Beavers to aban- don Horse Lake. The Hudson Bay Lowland is a flat coastal plain which is characterized by extensive peatland terrain. The peat is underlain by till, Tyrrell Sea marine de- posits and sedimentary bedrock. Limestone is the most common rock type and in places it directly underHes peat of variable thickness. One such area occurs along the Attawapiskat River centred about 90 km west of James Bay at 52°52'N, 83°44'W (Figure 2). This area has been referred to as the Attawapiskat Karst by Cowell(1981, 1983). The area consists of a complex of low limestone knolls representing ancient coral bio- herms. They are surrounded by a mature bog and fen complex having up to m of peat accumulation. Surface karst processes are concentrated at the inter- face between peatland and the reef knolls where numerous large sinkholes have developed. Subsurface capture at the sinkholes has lowered the water table in the surrounding peatlands slowing peatland develop- ment in the immediate area and initiating degradation of the organic blanket nearest to sinkpoints. The karst is very young (estimated to be less than 4400 years old) and many sinkholes drain completely only during the driest y
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