All of these examples of the relentless battle between constructive and destructive processes modifying the Teton landscape are but minor skirmishes. The bending and breaking of rocks at the surface are small reflections of enormous stresses and strains deep within the earth where the major conflict is being waged. It is revealed every now and then by a convulsion such as the 1959 earthquake in and west of Yellowstone Park. Events of this type release much more energy than all the nuclear devices thus far exploded by man. [Illustration: Figure 17. _Slide blocking main highway in northern part of Grand Teton National Park. National Park Service photo by Eliot Davis, May 1952._] ENORMOUS TIME AND DYNAMIC EARTH Framework of time One of geology’s greatest philosophical contributions has been the demonstration of the enormity of geologic time. Astronomers deal with distances so great that they are almost beyond understanding; nuclear physicists study objects so small that we can hardly imagine them. Similarly, the geologist is concerned with spans of time so immense that they are scarcely comprehensible. Geology is a science of time as well as rocks, and in our geologic story of the Teton region we must refer frequently to the geologic time scale, the yardstick by which we measure the vast reaches of time in earth history. Rocks and relative age Very early in the science of geology it was recognized that in many places one can tell the comparative ages of rocks by their relations to one another. For example, most _sedimentary rocks_ are consolidated accumulations of large or small rock fragments and were deposited as nearly horizontal layers of gravel, sand, or mud. In an undisturbed sequence of sedimentary rocks, the layer on the bottom was deposited first and the layer on top was deposited last. All of these must, of course, be younger than any previously formed rock fragments incorporated in them. _Igneous rocks_ are those formed by solidification of molten material, either as lava flows on the earth’s surface (_extrusive igneous rocks_) or at depth within the earth (_intrusive igneous rocks_). The relative ages of extrusive igneous rocks can often be determined in much the same way as those of sedimentary strata. A lava flow is younger than the rocks on which it rests, but older than those that rest on top of it. An intrusive igneous rock must be younger than the rocks that enclosed it at the time it solidified. It may contain pieces of the enclosing rocks that broke off the walls and fell into the liquid. Pebbles of the igneous rock that are incorporated in nearby sedimentary layers indicate that the sediments must be somewhat younger. All of these criteria tell us only that one rock is older or younger than another. They tell us little about the absolute age of the rocks or about how much older one is than the other.