The Morrison Formation Is Exposed in the Western United States

Multiple Choice

The Morrison Formation is exposed in the western United States and represents a nonmarine deposit with abundant dinosaur fossils. Much of the Morrison is pure mudstone, yet it is common to find highly polished pebbles and cobbles in the mud. Which of the following do you think is the best explanation of these cobbles?

A) They are impossible; people must have put them there.
B) They are gastroliths, a trace fossil.
C) They were placed in the mud by space aliens.
D) They represent meteorites that killed the dinosaurs.