How do geologists know that there once was a vast sea where the Grand Canyon is today?

Through the erosion and indentations. Formation down cutting it leaves forms of a river or ocean carved. Using range you can see how the floor of the canyon were an ocean floor. Erosion is caused by water the Grand Canyon sits on a body of water unlike most canyons this one happened to erode and sediment the body where the water once was. A layer of rock is also apparent which you wouldn’t be able to see unless erosion happened and etc.