At the close of the 18th century, the haze of fantasy and mysticism that tended to obscure the true nature of the Earth was being swept away. Careful studies by scientists showed that rocks had diverse origins. Some rock layers, containing clearly identifiable fossil remains of fish and other forms of aquatic animal and plant life, originally formed in the ocean. Other layers, consisting of sand grains winnowed clean by the pounding surf, obviously formed as beach deposits that marked the shorelines of ancient seas.
Certain layers of rock are in the form of sand bars and gravel banks - rock debris spread over the land by streams. Some rocks were once lava flows or beds of cinders and ash thrown out of ancient volcanoes; others are portions of large masses of once-molten rock that cooled very slowly far beneath the Earth's surface. Other rocks were so transformed by heat and pressure during the heaving and buckling of the Earth's crust in periods of mountain building that their original features were obliterated.
From the results of studies on the origins of the various kinds of rocks (petrology), coupled with studies of rock layering (stratigraphy) and the evolution of life (paleontology), today geologists reconstruct the sequence of events that has shaped the Earth's surface. Their studies show, for example, that during a particular episode the land surface was raised in one part of the world to form high plateaus and mountain ranges. After the uplift of the land, the forces of erosion attacked the highlands and the eroded rock debris was transported and redeposited in the lowlands.
During the same interval of time in another part of the world, the land surface subsided and was covered by the seas. With the sinking of the land surface, sediments were deposited on the ocean floor. The evidence of the pre-existence of ancient mountain ranges lies in the nature of the eroded rock debris, and the evidence of the seas' former presence is, in part, the fossil forms of marine life that accumulated with the bottom sediments.
Such recurring events as mountain building and sea encroachment, of which the rocks themselves are records, comprise units of geologic time even though the actual dates of the events are unknown. By comparison, the history of mankind is similarly organized into relative units of time. We speak of human events as occurring either B.C. or A.D. -broad divisions of time. Shorter spans are measured by the dynasties of ancient Egypt or by the reigns of kings and queens in Europe. Geologists have done the same thing to geologic time by dividing the Earth's history into Eras-broad spans based on the general character of life that existed during these times, and Periods-shorter spans based partly on evidence of major disturbances of the Earth's crust.
The names used to designate the divisions of geologic time are a fascinating mixture of works that mark highlights in the historical development of geologic science over the past 200 years. Nearly every name signifies the acceptance of a new scientific concept-a new rung in the ladder of geologic knowledge.
The major divisions, with brief explanations of each, are shown in the following scale of relative geologic time, which is arranged in chronological order with the oldest division at the bottom, the youngest at the top. Keyed to the relative time scale are examples of index fossils, the forms of life which existed during limited periods of geologic time and thus are used as guides to the age of the rocks in which they are preserved.
(Age of Recent Life)
|Quaternary Period||The several geologic eras were originally named Primary, Secondary, Tertiary, and Quaternary. The first two names are no longer used; Tertiary and Quaternary have been retained but used as period designations|
|Tertiary Period||The several geologic eras were originally named Primary, Secondary, Tertiary, and Quaternary. The first two names are no longer used; Tertiary and Quaternary have been retained but used as period designations|
(Age of Medieval Life)
|Cretaceous Period||Derived from Latin word for chalk (creta) and first applied to extensive deposits that form white cliffs along the English Channel.|
|Jurassic Period||Named for the Jura Mountains, located between Fance and Switzerland, where rocks of this age were first studied.|
|Triassic Period||Taken from the word "trias" in recognition of the threefold character of these rocks in Europe.|
Age of Ancient Life
|Permian Period||Named after the province of Perm, U.S.S.R., where these rocks were first studied.|
|Pennsylvanian Period||Named for the State of Pennsylvania where these rocks have produced much coal.|
|Mississippian Period||Named for the Mississippi River valley where these rocks are well exposed.|
|Devonian Period||Named after Devonshire, England, where these rocks were first studied.|
|Silurian Period||Named after Celtic tribes, the Silures and the Ordovices, that lived in Wales during the Roman Conquest.|
|Ordovician Period||Named after Celtic tribes, the Silures and the Ordovices, that lived in Wales during the Roman Conquest.|
|Cambrian Period||Taken from Roman name for Wales (Cambria) where rocks containing the earliest evidence of complex forms of life where first studied.|
|PRECAMBRIAN||---||The time between the birth of the planet and the appearance of complex forms of life. More than 80 percent of the Earth's estimated 4.5 billion years falls within this era.|
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