ancient rocks were deposited about 315 million years ago during Carboniferous time when great coal forming swamps existed in Pennsylvania.
(ancient rocks; were deposited; T:about 315 million years ago; T:during Carboniferous time) (great coal forming swamps; existed; L:in Pennsylvania; T:Carboniferous time)
ancient rocks great coal forming swamps
were deposited existed
T:about 315 million years ago T:during Carboniferous time
L:in Pennsylvania T:Carboniferous time