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Chapter 21

Across
The principle that fragments, called inclusions, in a rock layer must be older than the rock layer that contains them
The only indirect fossil evidence of an organism; traces of worm trails, footprints, and tunneling burrows
The theory that geologic processes occurring today have been occurring since Earth formed
Determines the age of relatively young organic objects; objects that are alive or were once living
The principle that an intrusion is younger than the rock it cuts across
Longest time unit in the geologic time scale
Fossil formed when an earlier fossil of a plant or animal leaves a cavity that becomes filled with minerals or sediment
Gap in the rock record caused by erosion or weathering
Unit of geologic time consisting of the first three eons during which Earth formed and became hospitable for life
Remains of plants or animals that were abundant, widely distributed, and existed briefly that can be used by geologists to correlate or date rock layers
Third-longest time unit in the geologic time scale, measured in tens of millions of years
Alternating light-colored and dark-colored sedimentary layer of sand, clay, and silt deposited in a lake that can be used to date cyclic events and changes in the environment
The principle that, in an undisturbed rock sequence, the oldest rocks are on the bottom and each consecutive layer is younger than the layer beneath it
Down
The principle that sedimentary rocks are deposited in horizontally or nearly horizontal layers
Describes a fossil with soft and hard parts that have undergone very little change since the organism's death
Fossil that can form when a shelled organism decays in sedimentary rock and is removed by erosion or weathering, leaving a hollowed-out impression
Establishing the order of past geologic events
Fossil whose organic material has been removed and whose hard parts have been changed by recrystallization or mineral replacement
Time unit in the geologic time scale, smaller than a period, measured in hundreds of thousands to millions of years
Second-longest time unit in the geologic time scale, measured in tens to hundreds of millions of years, defined by differences in life-forms that are preserved in rocks
A rock or sediment layer that serves as a time marker in the rock record and results from volcanic ash or meteorite-impact debris that spread out and covered large areas of Earth