Pickerel Lake formed on the Coteau des Prairies, northeastern South Dakota, prior to 10,670 ± 140 years ago. The lake was probably initially underlain by an ice block; it was insulated from the ice first by an undetermined thickness of outwash, and then also by sediment.
Fossil pollen, spores, seeds, leaves, and mollusks of a core from Pickerel Lake were analyzed in order to work out the limnological history of the lake and the vegetational history of the nearby upland. The data were then compared with ecological data from modern vegetation and the lake. The following chronology was deduced from these studies: (1) Prior to 10,670 years ago, boreal forest existed around Pickerel Lake and on the nearby upland, the climate was cool and moist, there were a few marshes around the lake margin, and the lake was less than 3 m deep at the coring site. The lake water was alkaline but not very hard. (2) Between 10,670 and about 8000 years ago, there were mixed deciduous trees around the lake, in gullies, and on the upland; however, tree cover on the upland was less dense and had numerous prairie-like openings with grass and perhaps bracken fern. The climate was warmer than that of the previous age. The lake was probably deeper at the study site, and was probably slightly harder. (3) From about 8000 to 4000 years ago, blue-stem prairie dominated the upland. Deciduous forest was almost absent, except perhaps for a few groves around the lake or in gullies. The climate was characterized by recurring summer drought, and the level of Pickerel Lake consequently fluctuated markedly. The depth of the lake at the coring site was less than 3 m, and the water was hard with high carbonate content. Reed marshes were common around the lake margin, and during low water levels a rich herbaceous vegetation invaded the exposed lake sediments. (4) Since about 4000 years ago, the upland and lake vegetation has been about the same as now, with prairie dominating the upland and with an abundant oak and ash deciduous forest common around lakes and in gullies. Summer droughts were not so common as during the preceding period. The lake was less than 3 m deep at the study site. It was also alkaline, hard, and had reed marshes flourishing in places around its margin.