Simplify the following text for a Grade 9 native English-speaking person.
Original: As a matter of fact most of the tuffs found in the older formations contain admixtures of clay , sand , and sometimes fossil shells , which prove that they were beds spread out under water .
Simplified: As a matter of fact most of the tuffs found in the older rock formations contain mixtures of clay , sand , and sometimes fossil shells , which prove that they were beds spread out under water .