Editing Till (section)

==Description==
Till is a form of ''[[Drift (geology)|glacial drift]]'', which is rock material transported by a [[glacier]] and deposited directly from the ice or from running water emerging from the ice.<ref>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=drift}}</ref> It is distinguished from other forms of drift in that it is deposited directly by glaciers without being reworked by meltwater.<ref name=Thornbury1969>{{cite book |last1=Thornbury |first1=William D. |title=Principles of geomorphology |date=1969 |publisher=Wiley |location=New York |isbn=0471861979 |edition=2d |page=379}}</ref>{{sfn|Jackson|1997|loc="till"}}<ref name=Boggs2006>{{cite book |last1=Boggs |first1=Sam |title=Principles of sedimentology and stratigraphy |date=2006 |publisher=Pearson Prentice Hall |location=Upper Saddle River, N.J. |isbn=0131547283 |edition=4th |page=281}}</ref><ref name=Allaby2013>{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=9780199653065 |edition=Fourth |chapter=till}}</ref>

Till is characteristically [[Sorting (sediment)|unsorted]] and [[Stratum|unstratified]], and is not usually [[Consolidation (geology)|consolidated]]. Most till consists predominantly of clay, [[silt]], and [[sand]], but with pebbles, cobbles, and boulders scattered through the till. The abundance of clay demonstrates lack of reworking by turbulent flow, which otherwise would [[Winnowing (sedimentology)|winnow]] the clay.<ref name=BlattEtal1980>{{cite book |last1=Blatt |first1=Harvey |last2=Middleton |first2=Gerard |last3=Murray |first3=Raymond |title=Origin of sedimentary rocks |date=1980 |publisher=Prentice-Hall |location=Englewood Cliffs, N.J. |isbn=0136427103 |edition=2d |page=372}}</ref> Typically, the distribution of particle sizes shows two peaks (it is ''bimodal'') with pebbles predominating in the coarser peak.<ref name=Boggs2006/>

The larger [[clasts]] (rock fragments) in till typically show a diverse composition, often including rock types from outcrops hundreds of kilometers away. Some clasts may be rounded, and these are thought to be stream pebbles entrained by the glacier. Many of the clasts are faceted, striated, or polished, all signs of glacial [[Abrasion (geology)|abrasion]]. The sand and silt grains are typically angular to subangular rather than rounded.<ref name=Boggs2006/>

It has been known since the careful statistic work by geologist Chauncey D. Holmes in 1941 that elongated clasts in tills tend to align with the direction of ice flow.<ref name=Evans2018>{{cite book |last1=Evans |first1=David J. A. |title=Glaciation : a very short introduction |date=2018 |location=Oxford, United Kingdom |isbn=9780198745853 |edition=First}}</ref> Clasts in till may also show slight [[Imbrication (sedimentology)|imbrication]], with the clasts dipping upstream.<ref name=Boggs2006/>

Though till is generally unstratified, till high in clay may show [[Lamination (geology)|lamination]] due to [[Compaction (geology)|compaction]] under the weight of overlying ice. Till may also contain lenses of sand or [[gravel]], indicating minor and local reworking by water transitional to non-till glacial drift.<ref name=Thornbury1969/>

The term ''till'' comes from an old [[Scottish people|Scottish]] name for coarse, rocky soil. It was first used to describe primary glacial deposits by Archibald Geikie in 1863.<ref>{{cite journal |last1=Geikie|first1=Archibald |title=On the Phenomena of the Glacial Drift of Scotland |journal=Transactions of the Geological Society of Glasgow |volume=1|number=2 |year=1863 |url=https://books.google.com/books?id=LLdn4jCtmu8C |access-date=11 May 2022}} Discussed in {{harvnb|Evans|2018|p=112}}</ref> Early researchers tended to prefer the term ''boulder clay'' for the same kind of sediments, but this has fallen into disfavor.{{sfn|Evans|2018|p=112}} Where it is unclear whether a poorly sorted, unconsolidated glacial deposit was deposited directly from glaciers, it is described as [[diamict]] or (when [[lithified]]) as [[diamictite]].<ref name=Boggs2006/> ''[[Tillite]]'' is a [[sedimentary rock]] formed by lithification of till.{{sfn|Jackson|1997|loc="tillite"}}