Sedimentary Sandstone Plains, plateaus, flanks of mountain
areas
Shale
Conglomerate
Limestone
Igneous Extrusive: Volcanic areas such as San Juan
Basalt Mountains, Spanish Peaks
Intrusive: Pikes Peak, Longs Peak, and most
Granite central mountain areas
Diorite
Metamorphic Marble Mountain areas
(from
limestone)
Quartzite
(from
sandstone)
Gneiss
(from
granite or
sandstone)
Schist
(from shale
or basalt)
Geologists arrange rocks in their chronologic sequence by studying the
fossils and minerals which they contain. The age of some rocks can be
determined with reasonable precision from ratios of radioactive
minerals and their fission products. The relative age of others can be
determined from their position, the fossils enclosed in them, and many
minor details of their structure.
The _stratigraphic column_ shown opposite may be thought of as a
calendar by which geologic events in Colorado can be arranged in their
proper order and related to events in the rest of the world.
Mississippian and Pennsylvanian Periods are American divisions;
elsewhere this time interval is known as the Carboniferous Period.
Other time terms are in worldwide use.
In the generalized geologic map of Colorado which accompanies Chapter
II, rocks are identified by the era in which they were formed. A more
detailed geologic map can be obtained from the U.S. Geologic Survey
map distribution center in the Federal Building, Denver.
[Illustration: Stratigraphic Column]
ERA Period Millions Distinctive fossils Events in Colorado
of years
ago
CENOZOIC
(Age of Mammals)
Quaternary Modern types of Development of present
animals and plants topography; glaciation in
mountains
3
Tertiary Mammals, flowering Uplift and mountain
plants building
70
MESOZOIC Dinosaurs and other
(Age of Reptiles) reptiles
Cretaceous Submergence, then uplift
135
Jurassic Desert, then submergence
180
Triassic Widespread floodplains
and deserts
225
PALEOZOIC
(Age of Fishes)
Permian First reptiles Widespread floodplains
and deserts
270
Pennsylvanian Swamp and forest “Ancestral Rocky
plants Mountains”
310
Mississippian Reef corals, sharks Partial submergence
350
Devonian Armored fish, first Probable submergence
insects
400
Silurian Corals and shellfish Probable submergence
440
Ordovician First fish Submergence
500
Cambrian First hard-shelled Gradual encroachment of
animals sea from west
570
PRECAMBRIAN “Lipalian Interval” Erosion to almost flat
surface or peneplain
Primitive Alternate episodes of
soft-bodied marine mountain building and
organisms erosion
3,600 plus