Wiktionary
n. 1 (context geography English) The highest cliff of a glacial cirque. 2 (context geology English) The steep cliff at the back of a cirque. 3 (context civil engineering English) A retaining wall at the outlet of a drain or culvert.
Wikipedia
In physical geography and geology the headwall of a glacial cirque is its highest cliff. The term has been more broadly used to describe similar geomorphic features of non-glacial origin consisting of a concave depression with convergent slopes typically of 65 percent or greater forming the upper end of a drainage valley.
In civil engineering, a headwall is a small retaining wall placed at the outlet of a stormwater pipe or culvert.
Usage examples of "headwall".
The moths aggregated in glacial cirques on talus right below steeper headwalls.
Once down the Headwall, it was possible to take the precipitous Sherburne Ski Trail back to the Notch and the highway.