James Hutton's Theory of the Earth: A Machine without a History - Time's Arrow, Time's Cycle: Myth and Metaphor in the Discovery of Geological Time - page 62

On us who saw these phenomena for the first time, the impression made will
not easily be forgotten . . . We often said to ourselves, What clearer evidence
could we have had of the different formation of these rocks, and of the long
interval which separated their formation, had we actually seen them emerging
from the bosom of the deep . . . Revolutions still more remote appeared in the
distance of this extraordinary perspective. The mind seemed to grow giddy
by looking so far into the abyss of time.
An unconformity is a fossil surface of erosion, a gap in time separating two
episodes in the formation of rocks. Unconformities are direct evidence that
the history of our earth includes several cycles of deposition and uplift.
I still use Hutton's drawing in my introductory courses to illustrate a principle
that continues to stun me with its elegance after twenty annual repetitions—
the complex panorama of history that can be inferred from the simple
geometry of horizontal above vertical, once you understand the basic rules
for deposition of strata. I can list a dozen distinct events that must have
occurred to produce this geometry, with Hutton's unconformity as the key.
Since large expanses of water-laid strata must be deposited flat (or nearly so),
the underlying vertical sequence arose at right angles to its current
orientation. These strata were then broken, uplifted, and tilted to vertical in
forming land above the ocean's surface. The land eroded, producing the
uneven horizontal surface of the unconformity itself. Eventually, the seas
rose again (or the land foundered) and waves further planed the old surface,
producing a "pudding stone" of pebbles made from the vertical strata. Under
the sea again, horizontal strata formed as products of the second cycle.
Another period of uplift then raised these rocks above the sea once more, this
time not breaking or tilting the strata. (Hutton reminds us that we must infer a
second episode of uplift by drawing a meeting of phaeton and solitary
horseman above the horizontal set of originally marine strata.) Thus, we see
in this simple geometry of horizontal above vertical two great cycles of
sedimentation with two episodes of uplift, the first tumultuous, the second
more gentle.