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Principle of inclusion
One of the basic principles of stratigraphy is the "principle of inclusion." The principle states that if fragments of a rock unit are found included in a second rock unit, the second unit is younger than the first. In this picture, fragments of the Mesoproterozoic Pikes Peak granite (like the clast on which the pencil is resting) are included in a conglomerate layer of the Pennsylvanian Fountain Formation, CO. We infer that the granite must have existed as a solid rock body that was broken and eroded before and during the deposition of the conglomerate layer. Therefore, this principle helps establishing a relative sequence of events. The principle was already understood by Nicolas Steno, the father of stratigraphy. In his "Prodromus," published in 1669, he writes: "these same bodies had already become hard at the time when the matter of the earth and rock containing them was still fluid. And not only did the earth and rock not produce the bodies contained in them, but they did not even exist as such when those bodies were produced in them” (pp. 15-16).
Marginocephalians
Marginocephalians are a group of herbivorous dinosaurs found in Cretaceous layers and characterized by a bony frill at the back of their skull. This photo shows specimens of 4 different species within this group, on display at the Field Museum of Natural History, Chicago.
CountyRd301
Cross-bedding, Co. Rd. 301 near McCoy --- About one third of the way up the picture are beds at an angle to the main layers.
Clastic dykes
Clastic dykes are sedimentary features consisting of seams of sediment truncating the surrounding host rock or deposit. These dykes can form through different processes. For example, fluidized sand can be injected through fractures, cutting through underlying and overlying deposits. Alternatively, pre-existing open cracks can be passively infilled by accumulation of a different kind of sediment. This picture shows three sub-vertical sandstone dykes, with parallel orientation, cutting through sub-horizontal mudstone and sandstone layers of the Pennsylvanian Fountain Formation, at the Balanced Rock site of Garden of the Gods Park, CO. Although different interpretations have been presented to explain the dykes in this outcrop (e.g., infill after thermal contraction of unconsolidated deposits in a cold environment), fieldwork by GRI scientists suggests that they should be interpreted as injection features, possibly related to seismic activation of small normal faults. Pencil for scale (resting vertically on right dyke).
Megatherium
Megatherium is one of the largest land mammals known to have existed and is part of the Pleistocene megafauna. This ground sloth was endemic to South America. Specimen on display at the Chicago Museum of Natural History
Rt13
Badlands, Wasatch Formation, Rt.13 north of Rifle --- The sandstone has been eroded into badlands and teepee-shaped mounds.
Cambrian quartzite
This sandstone, made up mostly of quartz grains, is an example of Cambrian quartzite. This type of deposit is very widespread in North America (and in other areas of the world), where it blankets the "Great Unconformity," the erosional surface marking the contact between crystalline basement and overlying sedimentary rocks. Sawatch Formation, vicinity of Colorado Springs, CO, USA
Fish and frond
A perfectly preserved fish resting by a perfectly preserved palm frond...This slab on display at the Field Museum of Natural History, Chicago, comes from Fossil Lake Basin, Wyoming, a locality that has yielded exquisite specimens of a variety of organisms. Special conditions are required for this kind of uncommon preservation.
ManyParksCurveStreamMeanders
WATER: Stream meanders, Horseshoe Park, Rocky Mountain National Park --- Water descending rapidly down a canyon forms a straight path, but when it arrives in an almost level valley it meanders as shown in this picture. The lower Mississippi River shows similar meanders.