STRATIGRAPHY AND HISTORICAL GEOLOGY

Some may revel alone in the wonders of the present scenery of the parks for others, however, this pleasure and inspiration may be greatly increased by the ability to perceive in each rocky ridge and profile, records of events which took place here millions of years ago when those rocks and profiles were in their formative stages. To repicture this story of the past and the agents which have operated to bring about the present state of things is to really understand and appreciate the meaning of scenery. The geological features of this region have long been under the careful scrutiny of many geologists and from their studies a great deal is known of its history, a history full of interesting events and shifting scenes. The characters of the present landscape are foreign to this drama of the past; they represent only a very recent shifting of the scenery. Recent in the geologist's language, for, although these features have been in existence for many hundreds of thousands of years, it is but a very brief interval as recorded by the geologist's clock. His only means of measuring time is by the rate of deposition of sediments, by the evolution of organisms or, more recently, by the use of the disintegration of radioactive minerals. The time concerned is of such length, and the yardstick always subject to so much correction, that time cannot be indicated in intervals less than millions of years. It is only in the latest of the geological epochs that events can be mere accurately dated, using here as a yardstick, thousands of years. It Is believed that the age of the earth is somewhere in the neighborhood of two billions of years old. The immensity of geologic time may be better grasped in the simile used by Jeans- "Let the height of the Woolworth building represent geologic time. We may then lay a nickel on its tower to represent the time of human existence. A thin sheet of paper on this will represent all historic time."

Just as human history is divided into various subdivisions on the basis of important events, so geological history has been subdivided into eras, periods, and epochs in accordance with the important events in the history of the earth. A chart showing the various divisions of geologic time and the names applied to them is presented.

Last Modified: Sat, Feb 9 2002 10:00:00 am PDT berkeley/effinger2/effinger2c.htm