They were made of bronze, bone, tooth and "Mud Cover" (picture 6), etc.

印材有铜印、骨印、牙印、陶印以及泥封（图六）等。

youdao

Troops moved forward block by block, hunting for cover behind crumbling mud-brick walls and houses.

CNN: 's Karl Penhaul

Hills that offer cool respite have lost significant green cover to overzealous building, leading to mud slides and, recently, flash floods.

CNN: Georgetown