to break into small fragments or crumbs.

to fall into small pieces; break or part into small fragments.

to decay or disintegrate gradually

The ancient walls had crumbled.

a crumbly or crumbled substance.

crumbles, bits of crisp bacon, bread, etc., added to other foods, esp. as a topping.

British Dialect . crumb; particle; fragment.

adjective

breaking up , disintegrating , collapsing , breaking , decaying