Geologic time is similarly organized, but it consists of four large units, called eras, that are based on characteristic life forms. Each time unit can be further subdivided into smaller units the divisions are approximations and do not have sharp boundaries. Historic time is sometimes divided into broad general time units characterized by prevalent attitudes or directions of growth, such as feudalism, medieval period, renaissance, etc. Utilizing these two methods, geologic events, ancient geography, environments, and their ages, can be determined from rock layers. A second tool of the geologist, recently developed, is the use of radioactive decay in determining the age of rocks. By finding modern environments where materials with the same characteristics are accumulating, it may be reasoned that ancient environments were similar to those of today. Each rock unit possesses a characteristic set of features, such as size, shape, and chemical composition of components. The first idea is not new, but it is one of the most fundamental tools in reconstructing events of the past. In developing a chronology of events in Pennsylvania’s geology, it may be helpful to explore several ideas that have revolutionized the field of geology. By compiling pieces of evidence, a geologic history of Pennsylvania can be constructed in much the same manner that written materials, legends, and artifacts can be combined to construct the history of man in Pennsylvania. Geology is a historical science that mirrors the events of the earth’s past, recorded in rock layers in various ways. the exploitive and economic interests of man are only one aspect of geology. Uranium deposits have also been found in the State and may one day provide a source of energy and economic growth. But iron, zinc, copper, gold and silver have also been produced. Pennsylvania minerals produced today include mostly coal, oil, natural gas, limestone (for cement, agriculture and construction), clay (for brick and tile), sand, and gravel. Pennsylvania ranks as the fifth largest mineral producer in the nation. In 1977, the value of minerals produced in the State amounted to about $3.2 billion. The impact of geology on Pennsylvania’s economy is currently about twice as great as that of agriculture. The fertility of its agricultural lands the design of its cities and transportation systems its water resources and the locations of wells, waste disposal sites, mines and quarries, all depend on geology. Geology has had a close association with the history of Pennsylvania. This article is a brief summary of the processes and events, as they are currently understood, that have molded our scenic Appalachian Mountains, carved the fertile valleys, and produced its geologic natural resources – a vital contribution to State and national economy. Equally fascinating is the geologic history of our State that attests to not only the past several thousand years, but the age of the earth itself, about four and one-half billion years. However, thousands of years before this virgin land saw its first European explorer, the earliest history of man in Pennsylvania was being recorded through artifacts created by its earliest residents. Its traditions have been recorded in the archives of printed materials, in the structures built by its people, and through the legends of individuals and events of the past. Its rich heritage in culture, industry, transportation, agriculture, education, communication, economic development, and many other areas of human endeavor have truly been a keystone in the forging of a growing nation. Pennsylvania and its citizens have made significant contributions to the development of America.
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