| Class | Example | Occurrence in Colorado |
|---|---|---|
| Sedimentary | Sandstone | Plains, [plateaus], flanks of mountain areas |
| Shale | ||
| [Conglomerate] | ||
| Limestone | ||
| Igneous | Extrusive: [Basalt] | Volcanic areas such as San Juan Mountains, Spanish Peaks |
| Intrusive: [Granite] [Diorite] | Pikes Peak, Longs Peak, and most central mountain areas | |
| Metamorphic | Marble (from limestone) | Mountain areas |
| 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.
Stratigraphic Column
| ERA Period | Millions of years ago | Distinctive [fossils] | Events in Colorado |
|---|---|---|---|
| CENOZOIC | |||
| (Age of Mammals) | |||
| Quaternary | Modern types of animals and plants | Development of present topography; [glaciation] in mountains | |
| 3 | |||
| Tertiary | Mammals, flowering plants | Uplift and mountain building | |
| 70 | |||
| MESOZOIC (Age of Reptiles) | Dinosaurs and other 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 plants | “Ancestral Rocky Mountains” | |
| 310 | |||
| Mississippian | [Reef] corals, sharks | Partial submergence | |
| 350 | |||
| Devonian | Armored fish, first insects | Probable submergence | |
| 400 | |||
| Silurian | Corals and shellfish | Probable submergence | |
| 440 | |||
| Ordovician | First fish | Submergence | |
| 500 | |||
| Cambrian | First hard-shelled animals | Gradual encroachment of sea from west | |
| 570 | |||
| PRECAMBRIAN | “Lipalian Interval” | Erosion to almost flat surface or [peneplain] | |
| Primitive soft-bodied marine organisms | Alternate episodes of mountain building and erosion | ||
| 3,600 plus | |||
THE PRAIRIES
Beneath the flat prairies of eastern Colorado, [sedimentary rocks] form a series of layers. Those near the surface are among the youngest rocks in Colorado. We know this from the [fossils] they bear, fossils of large mammals such as the hairy mammoth, which lived in early Quaternary time, the bison, and many smaller mammals living today.
The layers below—sandstones, shales, and limestones—become progressively older as one goes deeper. Most of them were formed originally on the bottoms of shallow seas that covered this part of North America several times during the history of the continent. In most places the layers are horizontal or nearly so, but westward, as they approach the mountains, they bend upward, gently at first and then more steeply. At the very edge of the mountains, where they were dragged upward when the mountains rose, their eroded edges appear at the surface.
The entire sequence of flat-lying rocks can be studied where they are exposed along the mountain front or where streams and rivers have dissected them. They are also known from cuttings and cores of oil and water wells. Some parts of Colorado’s eastern plains have been drilled so intensively in the search for oil and gas that we know a great deal about the subsurface sedimentary rock and can even make maps showing the distribution and character of the individual rock layers. From such maps, the history of the region can be deduced. We know, for example, that the area around Denver has subsided more in the past than has the area near La Junta or Lamar; it is called the Denver Basin because of its past history and not because it is a basin at present.
Although the plains of Colorado appear flat, they really slope gently eastward. The rock layers near the surface slope eastward also, but the deeper rock layers may not.