Editing Dune (section)

==Formation==
[[File:Saltation on sand dune.png|thumb|upright=1.25|Sand hitting sand is more likely to stick; sand hitting a more coherent surface is more likely to bounce ([[saltation (geology)|saltation]]). This exacerbating [[feedback loop]] helps sand accumulate into dunes.]]

A universally precise distinction does not exist between ripples, dunes, and [[#Draa|draas]],<ref name="Leeder2012">{{cite book|author=M.R. Leeder|title=Sedimentology: Process and Product|url=https://books.google.com/books?id=CUTvCAAAQBAJ&pg=PA97|date=6 December 2012|publisher=Springer Science & Business Media|isbn=978-94-009-5986-6|pages=97–}}</ref> which are all deposits of the same type of materials. Dunes are generally defined as greater than 7&nbsp;cm tall and may have ripples, while ripples are deposits that are less than 3&nbsp;cm tall.<ref name="PettijohnPotter2012">{{cite book|author1=F. J. Pettijohn|author2=P. E. Potter|author3=R. Siever|title=Sand and Sandstone|url=https://books.google.com/books?id=FFEyBwAAQBAJ&pg=PA346|date=6 December 2012|publisher=Springer Science & Business Media|isbn=978-1-4615-9974-6|pages=346–}}</ref> A draa is a very large aeolian landform, with a length of several kilometers and a height of tens to hundreds of meters, and which may have superimposed dunes.{{sfn|Jackson|1997|loc="Draa"}}

{{anchor|Slip face}}
Dunes are made of sand-sized particles, and may consist of quartz, calcium carbonate, snow, gypsum, or other materials. The upwind/upstream/upcurrent side of the dune is called the stoss side; the downflow side is called the lee side. Sand is pushed (creep) or bounces ([[saltation (geology)|saltation]]) up the stoss side, and slides down the lee side. A side of a dune that the sand has slid down is called a '''slip face''' (or slipface).

The [[Bagnold formula]] gives the speed at which particles can be transported.