headwall

noun3

1. •
   The highest cliff of a glacial cirque.
2. •
   The steep cliff at the back of a cirque.
3. •
   A retaining wall at the outlet of a drain, culvert or tunnel.

   • The platforms of all stations from Shepherds Bush to Liverpool Street have been lengthened from 325 ft. to 427 ft. between tunnel headwalls, to take longer trains. — 1950