Does the change from the thick gray shale to the overlying sandstone indicate a transgression or a regression

regression

Explanation:

Where the gray shale was deposited is the marine environment, then there is

sandstone and mudstone in the upper unit and they are deposited in a river system. And this can be regarded as a transition which exist between the Marine and non-marine environment which is as a result of relatively drop in level of sea and this is regarded as regression.

question

which are human systems?

the amazon rain forest

the appalachian trail

the george washington bridge

the himalayas

the hoover dam

the north american free trade agreement