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Abstract Wisconsinan full‐glacial silts filling a swale exposed in Conklin Quarry, Johnson Co., Iowa, contain a large and diverse biota that includes pollen, bryophytes, vascular‐plant macrofossils, small mammals, molluscs, and insects. Radiocarbon dates on wood from the top, middle and bottom of the swale fill respectively were 16710 ± 270, 17 170 ± 205, and 18090 ± 190 yr BP. The pollen diagram is dominated by Picea (spruce), Pinus (pine), and Cyperaceae (sedge), and it records low pollen accumulation rates. Plant macrofossils include a number of tundra species along with Picea and Larix (larch) needles and small pieces of wood. The insect fauna contains many species now confined to the forest‐tundra transition zone of northwestern Yukon and Alaska. Small mammals include the tundra indicators Dicrostonyx (collared lemming), and probably Microtus miurus (singing vole) together with boreal forest taxa. The molluscs include extinct and relict species and show the widest range in present geographic distribution, but Rocky Mountain and especially northern elements predominate in the swale fill. All these lines of evidence lead to consistent palaeoclimato‐logical interpretation and palaeoecological reconstruction. The dominant habitats represented by the biota and sedimentary environment collectively included open calcareous silty to sandy or gravelly upland sites, minerotrophic fens (wetlands), pond‐ or stream‐side clayey to sandy shores, and shallow (possibly ephemeral), cold, clear‐water ponds. Mean July temperatures were probably 11° to 13°C cooler than at present. The biota indicates that a Picea‐Larix krummholz with extensive tundra openings was present in southeastern Iowa between 18090 and 16710 yr BP.