"Deep time" refers to the time scale of geologic events, which is vastly, almost unimaginably greater than the time scale of human lives and human plans. It is one of geology's great gifts to the world's set of important ideas.
Deep Time and Religion
The concept of cosmology, the study of the origins and eventual fate of our universe, has been around as long as civilization itself. Before the advent of science, humans used religion to explain how the universe came into existence.
Many ancient traditions asserted that the universe is not only much larger than what we see but also much older. The Hindu series of yugas, for example, employs lengths of time so great as to be meaningless in human terms. In this way, it suggests eternity through the awe of large numbers.
On the opposite end of the spectrum, the Judeo-Christian Bible describes the history of the universe as a series of specific human lives, starting with "Adam begat Cain," between the creation and today. Bishop James Ussher, of Trinity College in Dublin, made the definitive version of this chronology in 1650 and announced that the universe was created starting in the evening of 22 October in 4004 BCE.
The biblical chronology was sufficient for people who had no need to concern themselves with geologic time. Despite overwhelming evidence against it, the literal Judeo-Christian creation story is still accepted as truth by some.
The Scottish geologist James Hutton is credited with exploding that young-Earth chronology with his painstaking observations of his farm fields and, by extension, the surrounding countryside. He watched the soil being washed into local streams and carried out to sea, and imagined it slowly accumulating into rocks like those he saw in his hillsides. He further supposed that the sea must exchange places with the land, in a cycle designed by God to replenish the soil, so that the sedimentary rock on the ocean floor could be tilted and washed away by another cycle of erosion. It was obvious to him that such a process, taking place at the rate he saw in operation, would take an immeasurable amount of time. Others before him had argued for an Earth older than the Bible, but he was the first to put the notion on a sound and testable physical basis. Thus, Hutton is considered the father of deep time, even though he never actually used the phrase.
A century later, the age of the Earth was widely considered to be some tens or hundreds of millions of years. There was little hard evidence to constrain speculation until the discovery of radioactivity and 20th-century advances in physics that brought about radiometric methods of dating rocks. By the mid-1900s, it was clear that Earth was about 4 billion years old, more than enough time for all of the geologic history we could envision.
The term "deep time" was one of John McPhee's most powerful phrases in a very good book, Basin and Range, first published in 1981. It first came up on page 29: "Numbers do not seem to work well with regard to deep time. Any number above a couple of thousand years-fifty thousand, fifty million-will with nearly equal effect awe the imagination to the point of paralysis." Artists and teachers have made efforts to make the concept of a million years accessible to the imagination, but it's hard to say that they induce enlightenment rather than McPhee's paralysis.
Deep Time in the Present
Geologists do not talk about deep time, except maybe rhetorically or in teaching. Instead, they live in it. They have their esoteric time scale, which they use as readily as common folk talk about their neighborhood streets. They use large numbers of years nimbly, abbreviating "million years" as "myr." In speaking, they commonly don't even say the units, referring to events with bare numbers.
Despite this, it's clear to me, after a lifetime immersed in the field, that even geologists can't really grasp geologic time. Instead, they have cultivated a sense of the deep present, a peculiar detachment in which it is possible for the effects of once-in-a-thousand-year events to be seen in today's landscape and for the prospect of rare and long-forgotten events to occur today.