The western boundary of Bethlehem Hospital as set out in Simon Fitz Mary's Charter of Foundation-to extend in length from the King's high street east to the great ditch in the west, the which is called "Deep Ditch" (Strype, ed. 1720, I. ii. 95 and Stow, ed. 1633).
It separated the Hospital from Moorfields (S. 166).
In Agas' map a stream is shown here flowing into the City ditch, which may be the remains of the Walbrook, the bed of which has been found under Blomfield Street, and might be referred to by Stow at that time as a ditch. It had been filled up in this part of its course, and had disappeared by 1658, the date of Faithorne's map, but was still shown in the same line further north as late as 1677 on O. and M. map. This ditch coincides with the City boundary (See Arch. lx. Pt. 1, p. 196 et seq.).
A Dictionary of London. Henry A Harben. 1918.