Pennsylvanian Period—323.2 to 298.9 MYA

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fossil tracks on sandstone slab
Pennsylvanian age fossil tracks, Grand Canyon National Park, Arizona.

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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

Some 300 million years ago the Rocky Mountain region of southern Wyoming, Colorado, and northern New Mexico was the site of intense tectonic activity, which culminated in the uplift of an ancient mountain chain that reached as much as 10,000 feet (3,050 m) of relief in some areas (Benedict 1991). Tectonic stability, which characterized earlier periods in this area, came to a dramatic halt during the Pennsylvanian Period as continental fragments began moving together to form the supercontinent of Pangaea.

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.
The first reptiles appeared during the Pennsylvanian Period. One of the earliest was the lizard-like Hylonomus, which was lightly built with deep, strong jaws and slender limbs. Several other major groups of reptiles appeared during the Pennsylvanian.

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.
By the Pennsylvanian Period, the evolution of terrestrial plants and animals had advanced to the point where true forests were developed in lowland, coastal sites. The presence of extensive, lush, swampy forests characterizes North America during the Pennsylvanian Period. These forests were composed of several types of trees that reproduced by spores. Lycophytes were the tallest plants of the Pennsylvanian, reaching heights up to 100 feet (30 m) and producing large amounts of plant biomass, which would later become coal. Lycopods still exist; however, modern relatives (e.g., club mosses) are small, generally only a few inches tall. Horsetails are another group of plants that are generally small today (up to 6 feet [1.8 m]), but during the Pennsylvanian, they reached heights up to 30 feet (9 m). Ferns were the third major non-seed plant group. Ferns are generally understory plants today, though in some tropical areas they can reach tree size. In the Pennsylvanian Period, ferns were present as trees (50 feet [15 m]), understory plants, and epiphytes (growing on other plants). The seed ferns were early seed plants that were a component of the forest understory. These trees had leaves that looked like ferns, but reproduced via large seeds (ferns reproduce with spores). Finally, early conifers made their appearance in the Pennsylvanian Period; the descendants of this ancient group of cone-bearing plants survive today as such familiar trees as fir, pine, redwood, and spruce.
At about 40 feet (12 m) thick, the coal beds of Pennsylvania are not nearly as thick as other coal deposits in the United States, such as the 1,400-foot- (430-m-) thick Paleocene-age beds of the Fort Union Group near Theodore Roosevelt National Park in North Dakota. Yet Pennsylvanian coal is iconic, bringing to mind huge trees in swampy forests. Coal beds reflect the transgression and regression of the seas over the continents of Europe and North America. Dead plants, which provided the organic material that became coal, did not completely decay and were instead turned to peat in these swamp forests. Peat is a porous, brown organic material in which twigs, roots, and other plant parts can still be recognized. When the sea covered these swamps, marine sediments covered the peat. Eventually, in a stepwise fashion from peat to lignite to bituminous coal (soft coal) or finally to anthracite coal (hard coal), heat and pressure transformed these organic remains. In each step of the transformation from green plant debris to hard coal, the percentage of carbon increases, until by the final step (hard coal) carbon makes up more than 90% of the mass. This material is a combustible fuel that is still a major source of energy for the United States.

Visit—Pennsylvanian Parks

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.

More about the Paleozoic

Last updated: September 11, 2020