The Geologist as Detective

THE PRINCIPLES USED IN DETERMINING THE ORDER IN WHICH ROCKS WERE FORMED

The idea that processes that shape the Earth, such as volcanoes, earthquakes, deposition and erosion, are the same processes that occurred in the geologic past

THE PRINCIPLE IS BASED ON THE FACT THAT SEDIMENT IS USUALLY DEPOSITED IN HORIZONTAL LAYERS

IF SEDIMENTARY ROCKS LIE AT AN ANGLE THEN SOME TECTONIC FORCES MUST HAVE ACTED ON THE ROCK

THAT SEDIMENTARY ROCKS ARE OLDEST ON THE BOTTOM AND YOUNGEST ON TOP

THIS IS TRUE AS LONG AS NO TECTONIC FORCES HAVE TURNED THE LAYERS UPSIDE DOWN OR ON THEIR SIDE

ROCK MUST FIRST EXIST BEFORE ANYTHING CAN HAPPEN TO IT

THEREFORE AN INTRUSION IS YOUNGER THAN THE ROCK IT HAS ENTERED

INTRUSION: WHEN MAGMA SQUEEZES INTO OR BETWEEN LAYERS OF PRE-EXISTING ROCK

EXTRUSION: WHEN MOLTEN ROCK FLOWS ONTO THE EARTH’S SURFACE, THEN COOLS

INCLUSION: OLDER ROCKS SURROUNDED BY MOLTEN MAGMA

ONE PIECE OF ROCK FOUND IN ANOTHER ROCK MUST BE OLDER THAN THE ROCK IT IS FOUND IN

A PEBBLE FOUND IN CONGLOMERATE IS OLDER THAN THE CONGLOMERATE

WHERE LAYERS OF ROCK ARE MISSING IF THE SURFACE OF THE ROCK WAS

ERODED SOME TIME IN ITS HISTORY UNCONFORMITIES ARE GAPS IN

GEOLOGICAL TIME

FOLDS: BENDS IN ROCK LAYERS PRODUCED BY CRUSTAL PLATE MOVEMENTS

FAULTS: BREAKS IN ROCK WHERE MOVEMENT HAS OCCURRED

FOLDING OR FAULTING CAN LEAD TO EXCEPTIONS TO THE LAW OF SUPERPOSITION

FOSSILS: PRESEVED REMAINS OR TRACES OF LIVING THINGS

RELATIVE AGE: COMPARATIVE AGE; AGE EXPRESSED AS BEFORE OR AFTER OTHER EVENTS WITHOUT SPECIFYING THE AGE IN UNITS

ABSOLUTE AGE:AGE OF A ROCK UNIT, FOSSIL OR EVENT EXPRESSED IN UNITS OF TIME, SUCH AS YEARS

Casts and Molds- shapes/outlines left behind; these are COPIES of originals.

Petrifaction- mineral replacement of organic material

Amber and Ice- organisms “frozen” in time

Trace Fossils- footprints, fecal pellets, burrows

Impressions- 2-Dimensional leftovers

NYS HAS FOSSILS OF CORAL AND WARM-WATER MARINE ORGANISMS, INDICATING THAT THE AREA WAS ONCE A TROPICAL SEA

IT ALSO HAS MORE RECENT FOSSILS OF WOOLY MAMMOTHS AND MASTODONTS, REVEALING THE COLD, SNOWY CLIMATE OF THE LAST ICE AGE