(earth/space science) (answer fast to get brainliest) why are some of the geological time periods not represented in the strata forming the grand canyon? a. the age of some of the grand canyon's layers is unknown. b. unconformities resulted in the erosion of some representative layers. c. during certain times of earth's past, sedimentary layers were not forming. d. most of the grand canyon's layers are made of igneous and metamorphic rocks.