Editing Drumlin (section)

== Formation ==
There are two main theories of drumlin formation.<ref>{{cite journal|title=Automated drumlin shape and volume estimation using high resolution LiDAR imagery (Curvature Based Relief Separation): A test from the Wadena Drumlin Field, Minnesota|doi=10.1016/j.geomorph.2015.07.020|volume=246|pages=589–601|journal=Geomorphology|bibcode=2015Geomo.246..589Y|year=2015|last1=Yu|first1=Peter|last2=Eyles|first2=Nick|last3=Sookhan|first3=Shane}}</ref> The first, ''constructional'', suggests that they form as sediment is deposited from subglacial waterways laden with till including gravel, clay, silt, and sand. As the drumlin forms, the scrape and flow of the glacier continues around it and the material deposited accumulates, the [[Clastic rock|clasts]]<ref>{{cite web|title=Clast Shape, Till Fabrics and Striae|url=http://www.antarcticglaciers.org/glacial-geology/techniques/clast-shape-till-fabrics-and-striae/#SECTION_2|access-date=2020-11-25|archive-date=2020-11-19|archive-url=https://web.archive.org/web/20201119102546/http://www.antarcticglaciers.org/glacial-geology/techniques/clast-shape-till-fabrics-and-striae/#SECTION_2|url-status=live}}</ref> align themselves with direction of flow.<ref>{{cite journal |last1=Hermanowski |first1=Piotr |last2=Piotrowski |first2=Jan A. |last3=Duda |first3=Piotr |title=Till kinematics in the Stargard drumlin field, NW Poland constrained by microstructural proxies |journal=Journal of Quaternary Science |date=October 2020 |volume=35 |issue=7 |pages=920–934 |doi=10.1002/jqs.3233 |bibcode=2020JQS....35..920H |s2cid=225275064 }}</ref> It is because of this process that geologists are able to determine how the drumlin formed using till fabric analysis, the study of the orientation and dip of particles within a till matrix.<ref>{{cite book |last1=Andrews |first1=John T. |title=Techniques of Till Fabric Analysis |date=1971 |publisher=Geo Abstracts |isbn=978-0-902246-04-1 }}</ref> By examining the till particles and plotting their orientation and dip on a stereonet, scientists are able to see if there is a correlation between each clast and the overall orientation of the drumlin: the more similar in orientation and dip of the clasts throughout the drumlin, the more likely it is that they had been deposited during the formation process. If the opposite is true, and there doesn't seem to be a link between the drumlin and the till, it suggests that the other main theory of formation could be true.{{Citation needed|date=July 2021}}

The second theory proposes that drumlins form by erosion of material from an unconsolidated bed. Erosion under a glacier in the immediate vicinity of a drumlin can be on the order of a meter's depth of sediment per year, depending heavily on the [[shear stress]] acting on the ground below the glacier from the weight of the glacier itself, with the eroded sediment forming a drumlin as it is repositioned and deposited.<ref name="Clark Hughes Greenwood et al 2009"/>

A hypothesis that catastrophic sub-glacial floods form drumlins by deposition or erosion challenges conventional explanations for drumlins.<ref>{{cite journal |last1=Shaw |first1=John |title=The meltwater hypothesis for subglacial bedforms |journal=Quaternary International |date=April 2002 |volume=90 |issue=1 |pages=5–22 |doi=10.1016/S1040-6182(01)00089-1 |bibcode=2002QuInt..90....5S }}</ref> It includes deposition of [[Glaciofluvial deposits|glaciofluvial sediment]] in cavities scoured into a glacier bed by subglacial meltwater, and remnant ridges left behind by erosion of soft sediment or hard rock by turbulent meltwater. This hypothesis requires huge, subglacial meltwater floods, each of which would raise sea level by tens of centimeters in a few weeks. Studies of erosional forms in bedrock at French River, Ontario, Canada, provide evidence for such floods.{{Citation needed|date=July 2021}}
[[File:Clew Bay, Ireland (ASTER).jpg|right|thumb|[[Clew Bay]] in [[County Mayo]], [[Ireland]], is a vast field of drowned drumlins, which are now [[List of islands of County Mayo|islands]].]]
The recent retreat of a marginal outlet glacier of [[Hofsjökull]] in Iceland<ref>A satellite image of the region of Hofsjökull where drumlin growth has been observed (see {{coord|64|39|25|N|18|41|41|W}}). The drumlins can be observed between pools of water.</ref> exposed a drumlin field with more than 50 drumlins ranging from {{convert|90|to(-)|320|m|ft|abbr=on}} in length, {{convert|30|to(-)|105|m|ft|-1|abbr=on}} in width, and {{convert|5|to(-)|10|m|ft|abbr=on}} in height. These formed through a progression of subglacial depositional and erosional processes, with each horizontal till bed within the drumlin created by an individual surge of the glacier.<ref name=B>{{Cite journal | doi = 10.1130/G31371.1 | last1 = Johnson | first1 = M. D. | last2 = Schomacker | first2 = A. | last3 = Benediktsson | first3 = I. O. | last4 = Geiger | first4 = A. J. | last5 = Ferguson | first5 = A. | last6 = Ingolfsson | first6 = O. | title = Active drumlin field revealed at the margin of Mulajokull, Iceland: A surge-type glacier | journal = Geology | volume = 38 | issue = 10 | pages = 943–946 | year = 2010|bibcode = 2010Geo....38..943J }}</ref> The above theory for the formation of these Icelandic drumlins best explains one type of drumlin. However, it does not provide a unifying explanation of all drumlins. For example, drumlin fields including drumlins composed entirely of hard bedrock cannot be explained by deposition and erosion of unconsolidated beds.<ref name="Lesemann Brennand 2009"/> Furthermore, hairpin scours around many drumlins are best explained by the erosive action of horseshoe vortices around obstacles in a turbulent boundary layer.<ref>{{cite journal |last1=Paik |first1=Joongcheol |last2=Escauriaza |first2=Cristian |last3=Sotiropoulos |first3=Fotis |title=On the bimodal dynamics of the turbulent horseshoe vortex system in a wing-body junction |journal=Physics of Fluids |date=1 April 2007 |volume=19 |issue=4 |pages=045107–045107–20 |doi=10.1063/1.2716813 |bibcode=2007PhFl...19d5107P }}</ref><ref>{{cite journal |last1=Shaw |first1=John |title=Hairpin erosional marks, horseshoe vortices and subglacial erosion |journal=Sedimentary Geology |date=June 1994 |volume=91 |issue=1–4 |pages=269–283 |doi=10.1016/0037-0738(94)90134-1 |bibcode=1994SedG...91..269S }}</ref>

Semi-submerged or drowned drumlins can be observed where rising sea-levels flooded the low-lying areas in between drumlin ridges. The most notable example of this is [[Clew Bay]] in the west of [[Ireland]], which contains hundreds of drumlin islands and islets. It was once a field of drumlins that was "drowned" following the [[Last Glacial Period]].<ref>{{cite web | title = Clew Bay | work = Mayo - County Geological Site Report | publisher = [[Geological Survey of Ireland]] | url = https://gsi.geodata.gov.ie/downloads/Geoheritage/Reports/MO032_Clew_Bay.pdf | date = 20 November 2020 | access-date = 29 December 2023 }}</ref>