What's the adjective for flattened? Here's the word you're looking for.
Included below are past participle and present participle forms for the
verbs flat and flatten which may be used as adjectives within certain contexts.
(phonetics)sonant; vocal, as distinguished from a sharp (non-sonant) consonant
(grammar) Not having an inflectional ending or sign, such as a noun used as an adjective, or an adjective as an adverb, without the addition of a formative suffix, or an infinitive without the sign "to".