Answer:
The Pennsylvanian age belongs to the lower Carboniferous period that started from 320 million years to 295 million years back. The environment of Nebraska including other places of North America during the Pennsylvanian environment was that there was the frequent changing of terrestrial to marine environment. The sea level was affected as the sea level has rised and dropped due to the melting of the glaciers in the geological past. A huge number of plants and trees have been buried under the surface during that time that led to the formation of most of the present day coals. The Pennsylvanian period provided a swampy environment that facilitates the coal formation as the sea level rised and covered these trees with the sediments.