Spume and a WW2 tank trap.

Spume - Foam, froth, frothy matter. 

From the Old French spume or the Middle English Spome. For example:

1390 J. Gower Confessio Amantis II. 265 Sche sette a caldron on the fyr,..And let it buile in such a plit, Til that sche sawh the spume whyt.