Editing Drumlin (section)

== Composition ==

Drumlins may comprise layers of [[clay]], [[silt]], sand, [[gravel]] and [[boulder]]s in various proportions; perhaps indicating that material was repeatedly added to a core, which may be of [[rock (geology)|rock]] or [[glacial till]]. Alternatively, drumlins may be residual, with the landforms resulting from erosion of material between the landforms. The [[Dilatancy (granular material)|dilatancy]] of glacial till was invoked as a major factor in drumlin formation.<ref>{{cite journal |last1=Smalley |first1=Ian J. |last2=Unwin |first2=David J. |title=The Formation and Shape of Drumlins and their Distribution and Orientation in Drumlin Fields |journal=Journal of Glaciology |date=1968 |volume=7 |issue=51 |pages=377–390 |doi=10.3189/S0022143000020591 |s2cid=129285660 |doi-access=free }}</ref> In other cases, drumlin fields include drumlins made up entirely of hard bedrock (e.g. [[granite]] or well-[[Limestone|lithified limestone]]).<ref name="Lesemann Brennand 2009">{{cite journal |last1=Lesemann |first1=Jerome-Etienne |last2=Brennand |first2=Tracy A. |title=Regional reconstruction of subglacial hydrology and glaciodynamic behaviour along the southern margin of the Cordilleran Ice Sheet in British Columbia, Canada and northern Washington State, USA |journal=Quaternary Science Reviews |date=November 2009 |volume=28 |issue=23–24 |pages=2420–2444 |doi=10.1016/j.quascirev.2009.04.019 |bibcode=2009QSRv...28.2420L }}</ref> These drumlins cannot be explained by the addition of soft sediment to a core. Thus, accretion and erosion of soft sediment by processes of subglacial deformation do not present unifying theories for all drumlins—some are composed of residual bedrock.{{citation needed|date=July 2018}}