Eras are divided into periods. Periods are divided into epochs

It's important to remember that an era is defined as a major division in geological time because each era represents a time when the earth's crust underwent , as another text says, a "profound disturbance and readjustment, resulting in broad continental uplifts, in the formation of ocean basins and in the concurrent deepening of the ocean basins. These are crustal revolutions."

A standard introduction to geology textbook states that:

"During the Mesozoic, subduction of the Pacific Plate along the western margin led to a series of orogenies that produced the Rocky Mountains and other mountain chains in the west....During the Cenozoic, parts of the west underwent tectonic movement. For example, uplift of the Colorado plateau led to the formation of the Grand Canyon, and subduction along Washington and Oregon formed the Cascade range."

But how slowly? How quickly? And, most important, by what mechanism?

The first set of events that produced the Rockies is estimated to have happened during the times of the dinosaurs, in the Mesozoic Era, starting about 225 millions years ago. The second set of events forming the Grand Canyon and the Cascades is estimated to have happened starting about 65 million years ago, in the Cenozoic Era.

Implicit in these brief time-lines, as signaled by the words Mesozoic and Cenozoic, is the million-year yardstick that allows these events to be thought of as more gradual, not abrupt. If there are presumed to be many millions of years between events, then it can more easily be assumed that each event probably took place over millions of years.

A thought: based on probabilities alone, how many sites of previous ice-capped poles are now under water, hiding their evidence. Given the amount of earth under water, that alone suggests a number.


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