Most drumlins — small, rounded hills — were formed under melting Pleistocene ice sheets. But the recent retreat of the Múlajökull glacier in Hofsjökull, Iceland, has uncovered the only known active drumlin field created during modern times.
Mark Johnson of the University of Gothenburg, Sweden and his colleagues identified more than 50 drumlins in this field and studied their sediment layers. Some theories posit that drumlins are carved out from the sediment by a rush of glacial meltwater. By contrast, the new observations suggest that drumlins form by sediment being repeatedly deposited and then eroded underneath the glacier over successive rounds of glacier growth and retreat.