(modified from Barnes, J. H.  and W. D. Sevon.  1996.)

Era or Eon

Period  (years ago)

Activity Affecting Pennsylvania

Main PA deposits

Dominant Life in PA

Cenozoic Era

(Recent Life)


(0 to 2 million)

Glaciation; periglacial erosion and deposition

Sand, silt, clay, gravel

Mammals, including humans


(2 to 67 million) 

Weathering and erosion.  Creation of present landscape

Sand, silt, gravel

Mammals, grasses

Mesozoic Era

(Middle Life)


(67 - 140 million)

Erosion and weathering

Clay, Sand

Dinosaurs, mammals, birds


(140 - 208 million)

Diabase intrusions; opening of Atlantic Ocean


Dinosaurs, mammals, birds


(208-250 million)

Separation of North America from Africa; sedimentation in rift valley

Shale, sandstone, diabase

Dinosaurs, early mammals and birds

Paleozoic Era


(250-290 million)

Allehanian Orogeny:  Collision of Africa and North America; mountain building, thrust faulting and folding; much erosion

Sandstone, shale

Insects, amphibians, reptiles

Pennsylvanian and Mississippian (Carboniferous)

(290-365 million)

Alluvial deposition; eastward advance of shoreline followed by development of low, flat alluvial plain

Sandstone, siltstone, shale, coal, limestone

Trees, ferns, amphibians, air-breathing mollusks, insects


(365-405 million)

Acadian Orogeny; Collision of Avalonia, Europe, and North America; formation of Catskill Delta

Conglomerate, sandstone, shale

Fish, amphibians, insects, land plants 


(405-430 million)

Erosion of mountains; deposition of sand and mud

Conglomerate, sandstone, limestone

Corals, fish


(430-500 million)

Taconic Orogeny:  Thrusting of volcanic arc; development of Appalachian basin

Shale, limestone, dolomite

Mollusks, bryozoans, graptolites


(500-570 million)

Transgression of the sea' carbonate deposition

Limestone, dolomite, quartzite

Trilobites, brachiopods

Proterozoic Eon

(570 million-2.5 billion)

Accretion of microplates to form Laurentia

Schist, slate, marble

Blue-green algae, jellyfish, worms

Archean Eon

(2.5-3.8 billion)

Bombardment by meteorites; creation of continental crust

None identified


Pre-Archean Eon

(3.8-4.5 billion)

Formation of Earth and solar system

None identified

None identified