Originally any deep hole or pit, the current meaning of a small lush valley is related to “dale”.

• 06.004 and there were dells below the level of the path overhung with bushes
• 06.004 in one of these dells under the bushes people were talking.
• 06.006 as he crawled into the bushes at the edge of the dell.
• 13.064 and before long came to a deep dell

“dell, n.1.” OED Online. Oxford University Press, June 2015. Web. 27 July 2015.