The Pennsylvanian subperiod is named for the state of Pennsylvania. In 1891 Henry Shaler Williams coined the name for the younger strata of the Carboniferous Period that are well exposed in Pennsylvania. These rocks serve as a counterpart to the previous geologic period—the Mississippian. In North America, the Mississippian and Pennsylvanian periods are used instead of the Carboniferous Period, which covers that same time frame in other parts of the world.
Significant Pennsylvanian events
In general, the rocks of the Pennsylvanian subperiod can be distinguished from those of the older Mississippian by their terrestrial nature. In North America, seas covered parts of the continent during the Mississippian subperiod. As a result, most of the rocks found from Mississippian time are marine limestone. The uplift of the continent, which resulted in the Ancestral Rocky Mountains, caused transition to a more extensive terrestrial environment during the Pennsylvanian Period. Swamp forests and terrestrial habitats became more common and widespread. Therefore, Pennsylvanian strata contain more sandstone and shale (terrestrial) layers than Mississippian strata.
During the Pennsylvanian Period, adaptations occurred in animals and plants that allowed for reproduction on dry land. In the case of plants, the adaptation was the further evolution of the seed, which first appeared in the Devonian Period. In the case of animals, it was the amniotic egg—a key feature in the origin of reptiles. In both cases, these adaptations severed the ties to water, allowing for life-long terrestrial habitation.
The vast amount of plant material provided by extensive forests resulted in the later formation of coal, which characterizes the Pennsylvanian Period. Also characteristic of Pennsylvanian time was the diversification of amphibians and insects. For example, insects with non-folding wings appeared. Some of these insects grew to very large sizes; for example, some dragonflies had wingspans of up to 2.5 feet (0.8 m), and some cockroaches were a foot long. Millipedes, scorpions, and spiders also became significant inhabitants of the lush forests.
Learn more about events in the Pennsylvanian Period
During the early stages of tectonic activity, shallow seas invaded the region from the southeast, depositing marine sediments in the basins between the mountain uplifts. The most important of these basins was a deep, narrow trough known as the Central Colorado Trough. It extended from northwestern to south-central Colorado. Between 10,000 and 20,000 feet (3,050 and 6,100 m) of marine and terrestrial strata accumulated within the trough; these rocks record a long history of the sea migrating inland.
Within the center of the Central Colorado Trough, thick sequences of interbedded gypsum, anhydrite, halite, black shale, and siltstone accumulated. These evaporite rocks formed in a warm, arid climate and precipitated from the brackish waters left behind by the evaporation of the narrow seaway (Benedict 1991).
Along the flanks of the Ancestral Rockies, sediments that eroded from the granitic mountains accumulated as valley fill, braided alluvial (water-deposited) plains, and a series of coalescing alluvial fans (also called bajadas). Thick sequences (from 7,000 to 12,000 feet [2,100–3,660 m]) of these non-marine sediments, consisting mostly of feldspar-rich gravels and sand, consolidated to form colorful red beds in many areas of the Southern Rockies. This thick sequence of rock, as much as 12,000 feet (3,660 m) thick, is the Fountain Formation. In Colorado, the Fountain Formation comprises the flatirons of Boulder; Red Rocks Park, near Denver; and Garden of the Gods in Colorado Springs.
A key feature in the origin of reptiles is the development of the amniotic egg, which eliminated the need to lay eggs in water. This allowed vertebrates to reproduce on dry land for the first time. Like today’s chicken eggs, this new type of egg had a durable outer shell for protection. The “white” part prevented drying while still permitting air to reach the embryo. The yolky part provided nutrition, which reduces in size as the embryo matures. A series of fluid-filled membranes permitted the embryo to survive: one sac contained the embryo in a stable, fluid environment; another sac provided for gas diffusion and the removal of waste products.
Other features that helped the reptiles survive on land were scaly skin, which solved the problem of drying out even in intense heat, and the change in orientation of the legs in many species. This change in locomotion resulted in an efficient running style, unlike the ungainly waddling of amphibians. Because of these features reptiles adapted to terrestrial life better than amphibians, which may be the reason they largely displaced them in abundance.
Every park contains some slice of geologic time. Below, we highlight selected parks associated with the Pennsylvanian Period. This is not to say that a particular park has only rocks from the specified period. Rather, rocks in selected parks exemplify a certain event or preserve fossils or rocks from a certain geologic age.
- Bighorn Canyon National Recreation Area, Montana and Wyoming—[Geodiversity Atlas] [Park Home]
- Big South Fork National River and Recreation Area, Kentucky and Tennessee—[Geodiversity Atlas] [Park Home]
- Canyonlands National Par, Utahk—[Geodiversity Atlas] [Park Home]
- Chickasaw National Recreation Area, Oklahoma—[Geodiversity Atlas] [Park Home]
- Obed Wild and Scenic Rive, Tennessee—[Geodiversity Atlas] [Park Home]
- Parashant National Monument, Arizona—[Geodiversity Atlas] [Park Home]
More about the Paleozoic
Series: Geologic Time Periods in the Paleozoic Era
Last updated: January 4, 2021