Two large drumlin fields in central Finland

The glacial morphology of the large Pieksämäki and Keitele drumlin fields, covering about 25 000 sq. km. in central Finland, has been studied. All the 14500 drumlins in the area have been mapped and classified into five morphogenetical types: rock drumlins, rock‑end drumlins, typical drumlins, drumloids and drumlin shields. In the flat central parts of the fields there are typical drumlins, while in their surroundings, where the relief is uneven and rocky, there are rock drumlins and incompletely developed drumloids. During the advance towards SE of the active moving continental ice sheet the drumlins were formed subglacially through re‑shaping of basal till into streamlined ridges around a rock core. The influence of the main fracture zones of the bedrock is reflected both in the longitudinal streamlined structure and in the positioning of the drumlins along transverse arcs in the fields. About 15 discontinuous esker systems run across the drumlin fields from S–SE to N–NW. During deglaciation, in the marginal zone of the retreating melting ice, the eskers were deposited on the ground moraine and the drumlins. The run of the eskers proves that the direction of recession of the ice differs from that of the ice advance.