Drumlin location as a response to bedrock topography on the southeastern slope of the Fennoscandian Shield

There are several thousand drumlins in Estonia, northern Latvia and on the bottom of the Gulf of Finland. Drumlins occur most frequently behind bedrock elevations, on the slopes of which they form huge crag-and-tail formations, and in marginal areas of depressions where the monolithic glacier disintegrated into tongues which moved at different velocities. A dense net of boreholes and detailed geophysical measurements showed that even small irregularities of the bedrock topography and slight variations in the resistance of bedrock to glacial erosion exerted remarkable influence upon the location, shape and internal structure of drumlins and drumlin-like forms. This accounts for the occurrence of drumlins with distinctly different structures within the same drumlin field. However, the drumlins were formed in a variety of ways and at different times under the effect of glaciers with different thickness. In most cases they seem to have formed near the ice margin under rather thin glacier ice during its rapid advance when the dynamics of the ice responded to topographical peculiarities.