THE CAVALRY HORSE AND HIS PACK
CHAPTER 1 GEOLOGIC TIME CHAPTER 2 DATING CHAPTER 3 PRECAMBRIAN TIME CHAPTER EOLOGIC TIME SPANS EARTH S GEOLOGIC HISTORY IT EXTENDS FROM ABOUT 4 6 BILLION YEARS AGO CORRESPONDING TO THE AGE OF EARTH S FORMATION TO THE PRESENT DAY SOME SCI ENTISTS MAINTAIN HOWEVER THAT GEOLOGIC TIME BEGINS WITH THE OLDEST KNOWN ROCKS THAT WERE CREATED SOME 4 4 BILLION THE GEOLOGIC TIME SCALE IS THE CALENDAR FOR EVENTS IN EARTH HISTORY IT SUBDIVIDES ALL TIME SINCE THE END OF THE EARTH S FORMATIVE PERIOD AS A PLANET NEARLY 4 6 BILLION YEARS AGO INTO NAMED UNITS OF ABSTRACT TIME THE LATTER IN DESCENDING ORDER OF DURATION ARE EONS ERAS PERIODS AND EPOCHS THE ENUMERATION OF THESE GEOLOGIC TIME UNITS IS BASED ON STRATIGRAPHY WHICH IS THE CORRELATION AND CLASSIFI CATION OF ROCK STRATA THE FOSSIL FORMS THAT OCCUR IN THESE ROCKS PROVIDE THE CHIEF MEANS OF ESTABLISHING A GEOLOGIC TIME SCALE BECAUSE LIVING THINGS HAVE UNDERGONE EVOLU TIONARY CHANGES OVER GEOLOGIC TIME PARTICULAR KINDS OF ORGANISMS ARE CHARACTERISTIC OF PARTICULAR PARTS OF THE GEO LOGIC RECORD BY CORRELATING THE STRATA IN WHICH CERTAIN TYPES OF FOSSILS ARE FOUND THE GEOLOGIC HISTORY OF VARIOUS REGIONS AND OF THE EARTH AS A WHOLE CAN BE RECONSTRUCTED THE RELATIVE GEOLOGIC TIME SCALE DEVELOPED FROM THE FOSSIL RECORD HAS BEEN NUMERICALLY QUANTIFI ED BY MEANS OF ABSO LUTE DATES OBTAINED WITH RADIOMETRIC DATING METHODS GEOCHRONOLOGY IS THE FI ELD OF SCIENTIFI C INVESTIGATION CON CERNED WITH DETERMINING THE AGE AND HISTORY OF THE EARTH S ROCKS AND ROCK ASSEMBLAGES SUCH TIME DETERMINATIONS ARE MADE AND THE RECORD OF PAST GEOLOGIC EVENTS IS DECI PHERED BY STUDYING THE DISTRIBUTION AND SUCCESSION OF ROCK STRATA AS WELL AS THE CHARACTER OF THE FOSSIL ORGANISMS PRESERVED WITHIN THE STRATA THE EARTH S SURFACE IS A COMPLEX MOSAIC OF EXPOSURES OF DIFFERENT ROCK TYPES THAT ARE ASSEMBLED IN AN ASTONISH ING ARRAY OF GEOMETRIES AND SEQUENCES INDIVIDUAL ROCKS IN THE MYRIAD OF ROCK OUTCROPPINGS OR IN SOME INSTANCES SHALLOW SUBSURFACE OCCURRENCES CONTAIN CERTAIN MATERIALS OR MINERALOGIC INFORMATION THAT CAN PROVIDE INSIGHT AS TO FOR YEARS INVESTIGATORS DETERMINED THE RELATIVE AGES OF SEDIMENTARY ROCK STRATA ON THE BASIS OF THEIR POSITIONS IN AN OUTCROP AND THEIR FOSSIL CONTENT ACCORDING TO A LONG STANDING PRINCIPLE OF THE GEOSCIENCES THAT OF SUPERPOSITION THE OLDEST LAYER WITHIN A SEQUENCE OF STRATA IS AT THE BASE AND THE LAYERS ARE PROGRESSIVELY YOUNGER WITH ASCENDING ORDER THE RELATIVE AGES OF THE ROCK STRATA DEDUCED IN THIS MANNER CAN BE CORROBORATED AND AT TIMES REFINED BY THE EXAMINATION OF THE FOSSIL FORMS PRESENT THE TRACING AND MATCHING OF THE FOSSIL CONTENT OF SEPARATE ROCK OUTCROPS THAT IS THE CORRELATION PROCESS EVENTUALLY ENABLED INVES TIGATORS TO INTEGRATE ROCK SEQUENCES IN MANY AREAS OF THE WORLD AND CONSTRUCT A RELATIVE GEOLOGIC TIME SCALE SCIENTIFIC KNOWLEDGE OF THE EARTH S GEOLOGIC HISTORY HAS ADVANCED SIGNIFICANTLY SINCE THE DEVELOPMENT OF RADIO METRIC DATING A METHOD OF AGE DETERMINATION BASED ON THE PRINCIPLE THAT RADIOACTIVE ATOMS IN GEOLOGIC MATERIALS DECAY AT CONSTANT KNOWN RATES TO DAUGHTER ATOMS RADIOMETRIC DATING HAS PROVIDED NOT ONLY A MEANS OF NUMERICALLY QUANTIFYING GEOLOGIC TIME BUT ALSO A TOOL FOR DETERMINING THE AGE OF VARIOUS ROCKS THAT PREDATE THE APPEARANCE OF LIFE FORMS THE CAVALRY HORSE AND HIS PACK EARLY VIEWS AND DISCOVERIES SOME ESTIMATES SUGGEST THAT AS MUCH AS 70 PERCENT OF ALL ROCKS OUTCROPPING FROM THE EARTH S SURFACE ARE SEDIMEN TARY PRESERVED IN THESE ROCKS IS THE COMPLEX RECORD OF THE MANY TRANSGRESSIONS AND REGRESSIONS OF THE SEA AS WELL AS THE FOSSIL REMAINS OR OTHER INDICATIONS OF NOW EXTINCT ORGANISMS AND THE PETRIFIED SANDS AND GRAVELS OF ANCIENT BEACHES SAND DUNES AND RIVERS THIS ANCIENT DRAGONFLY WAS FOSSILIZED IN LIMESTONE MANY SPECIES OF DRAGON FLIES LIVED DURING THE PRECAMBRIAN ERA COLIN KEATES DORLING KINDERSLEY