Sand boils are springs that form on the lowland side of an artificial levee containing a river at extremely high flood stage. Hydrostatic pressure generated by the column of river water between the levees causes failure in the sediment of the channel wall and allows water to be forced laterally beneath the levee and out onto the adjacent flood plain. Sand is transported by the moving water and is ejected onto the flood plain at points where the sediment is structurally weak to produce the boils. The sand deposit forms a characteristic sedimentary structure similar to sand volcanos of the ancient sedimentary record. Mechanisms similar to those that produce sand boils may have been involved in the genesis of these structures.
Williams, John Glynn
"Sand Boils: A Modern Analogue of Ancient Sand Volcanoes,"
Journal of the Arkansas Academy of Science: Vol. 28
, Article 28.
Available at: http://scholarworks.uark.edu/jaas/vol28/iss1/28