Paleontology and geology
The Precambrian: Precambrian rocks in Maryland are metamorphic and igneous. No fossils have been positively identified from these rocks thus far.
The Paleozoic: The Paleozoic in Maryland was a time of changing sea levels. Episodes of deposition in shallow seas were interrupted by mountain-building events and subsequent periods of erosion. Cambrian and Ordovician sedimentary rocks contain abundant marine fossils, indicating the presence of warm, shallow seas. A period of uplift and mountain building during the Late Ordovician was followed by a time of erosion and subsidence and the shallow seas returned. Two additional episodes of mountain building and subsequent erosion and subsidence followed, first in the Devonian and then at the end of the Paleozoic (Carboniferous and Permian). During times of subsidence, the seas returned and flooded the lands. These cycles of uplift and flooding led to alternating deposition of non-marine and marine sediments. Brachiopods and bryozoans were abundant in the sea, while horsetail rushes and scale trees thrived in the warm, humid, Late Paleozoic climate on land. Most of Maryland lay above sea level by the end of the Paleozoic.
The Mesozoic: Rift basins developed in Maryland during the early and middle Mesozoic as plate movement pulled apart the supercontinent of Pangea. Sediments washed into these valleys and accumulated as lake and river deposits. Fossils of plants and dinosaur footprints can be found in these rocks. Late in the Mesozoic (Cretaceous), seas advanced and retreated across the coastal plain numerous times, providing habitats that ranged from terrestrial to deep continental shelf. Fossils of plants and dinosaurs are common in the terrestrial deposits, while marine fossils include molluscs and brachiopods.
The Cenozoic: During the Early Cenozoic (Tertiary), sea level fluctuated in eastern Maryland, leaving marine deposits in the Chesapeake Bay region that are famous for their beautifully preserved fossils. The climate cooled later in the Cenozoic (Quaternary), and vast ice sheets advanced and retreated across North America. Although the ice sheets never reached Maryland, sea levels dropped as the glaciers expanded. The cooler temperatures encouraged the growth of new types of plants, while mastodons and other cold-adapted animals roamed the state. During warmer climate periods (interglacials), sea levels again rose to cover the eastern part of the state. Fossils of marine clams, oysters, and snails are abundant in the rocks formed during these interglacial episodes.