Use the geologic time scale to comprehend geologic time, read the fossil record and past climate changes from rock, and interpret Ohio's bedrock and glacial history

Learning goal: Describe the divisions of the geologic time scale and major events within them; make the length of geologic time comprehensible with scale models; explain how the fossil record forms, what it is biased toward and how index fossils correlate rocks; identify climate changes recorded in rock; interpret Ohio's bedrock record and its glacial history, comparing northern and southern Ohio; relate past climate change to present evidence; and design a field study that combines fossil and rock evidence to interpret local history.

Where this lesson sits. Ohio's PG.EH.1 (The geologic rock record) is taught on this site in two lessons meant to be studied together. This lesson is part 2 of 2: the geologic time scale and its divisions, comprehending geologic time, climate changes evident in the rock record, the fossil record, and Ohio's bedrock and glacial history, with a local field study and a 3-D model. Part 1, the lesson on relative and absolute age, teaches the principles of relative age, radiometric dating (how an age is calculated, which isotope to choose and why isotopes can disagree) and how relative and absolute ages are combined. Neither part alone covers PG.EH.1.

Dividing geologic time. Earth formed about 4.6 billion years ago. The geologic time scale divides that span into eons, eras, periods and epochs. The divisions were first drawn from changes in fossils and rock, using relative dating, and radiometric dates were added later, so boundaries often mark major changes in life. The Hadean, Archean and Proterozoic eons (together often called Precambrian time) span about 4.6 billion to 539 million years ago, almost 90% of Earth history; they include the oldest rocks and minerals, the first single-celled life by at least 3.5 billion years ago, the build-up of oxygen in the atmosphere, and the first multicellular organisms. The Phanerozoic Eon, "visible life", began about 539 million years ago with the rapid appearance of many animal groups with hard parts. It has three eras. The Paleozoic (about 539 to 252 million years ago) saw marine invertebrates, the first fish, land plants, amphibians and reptiles, and ended with the largest mass extinction known. The Mesozoic (about 252 to 66 million years ago) was the age of dinosaurs, the first mammals and birds and flowering plants, and it ended with the mass extinction linked to an asteroid impact. The Cenozoic (66 million years ago to today) saw mammals diversify, and its most recent part includes the ice ages and the appearance of modern humans, about 300,000 years ago.

Comprehending geologic time. Numbers in the billions are hard to feel, so scale them. Compress 4.6 billion years into one 24-hour day. Each hour represents about 192 million years; each second about 53,000 years. The Phanerozoic begins at about 9:11 p.m.; the dinosaurs die out at about 11:39 p.m.; the whole existence of modern humans, 300,000 years, takes the last 5.6 seconds before midnight. Here is the arithmetic for the dinosaur extinction: 66 ÷ 4,600 = 0.01435 of the day; 0.01435 × 1,440 minutes = 20.7 minutes before midnight, which is about 11:39 p.m. The point is that most of Earth's history happened before complex life, and all of recorded human history is a small fraction of the last second.

The fossil record. A fossil is any preserved evidence of past life: shells, bones, teeth, wood, and also traces such as footprints and burrows. Preservation is rare and biased. Organisms with hard parts (shells, bones) are preserved far more often than soft ones; those buried quickly in fine sediment, especially in water with little oxygen, are more likely to survive; and organisms living where sediment accumulates, in seas and lakes, are over-represented compared with those on uplands where erosion dominates. The record is therefore a sample, not a census. Still, the order of fossils is consistent worldwide (faunal succession), which makes index fossils powerful. An index fossil is abundant, widespread, easy to recognize and existed for only a short span of time; finding it in two distant rock layers shows the layers are about the same age, correlating them. Trilobites, graptolites and ammonites are classic index groups.

Climate change in the rock record. Rocks and fossils record past climates when read with present-day conditions as the key. Coal records wet swamps (tropical when its plant fossils are tropical types); salt and gypsum record evaporation exceeding inflow, usually in hot, dry climates; reef limestone built by warm-water reef organisms records warm, shallow seas; red, cross-bedded dune sandstones record deserts; tillites, lithified glacial deposits with scratched boulders, record ice; and ratios of oxygen isotopes in shells and in ice cores track global temperature and ice volume. Plate motion complicates the reading: a place's climate changes both because global climate changes and because the continent moves through different latitudes. Ohio's Pennsylvanian coals formed when this part of North America lay near the equator.

Ohio's rock record. Ohio's bedrock at the surface is sedimentary rock of the Paleozoic Era, from the Ordovician Period in the southwest to the Dunkard Group in the southeast, whose exact age, latest Pennsylvanian or early Permian, geologists have not been able to settle. In southwestern Ohio the Ordovician limestones and shales around Cincinnati are among the richest fossil beds in North America: brachiopods, bryozoans, crinoids, horn corals and trilobites, including Isotelus, Ohio's official state invertebrate fossil, all animals of a warm, shallow sea. Silurian dolostones and salt record shallow seas, some evaporating in a dry climate. Devonian rocks include the Columbus Limestone with its fossil corals and the black Ohio Shale; the Cleveland Shale part of it yields the armored fish Dunkleosteus. Mississippian sandstones record deltas and shorelines. Pennsylvanian rocks repeat cycles of coal, shale, limestone and sandstone as seas advanced and retreated over coastal swamps, with fossil ferns and tree-sized relatives of club mosses. Then there is a gap: in Ohio no bedrock records the almost 300 million years between the Dunkard Group (about 300 million years old, at the Pennsylvanian–Permian boundary) and the ice ages. The ice-age deposits rest directly on Paleozoic rock, a major unconformity.

Ohio's glacial record. During the ice ages of roughly the last two million years, continental ice sheets advanced from Canada several times and covered about two-thirds of Ohio: the north and west. The last ice had melted out of the state by about 14,000 years ago. A glacial map shows a boundary running from near Cincinnati, north around the Hocking Hills, to the Pennsylvania line in the east. North and west of it, ice left till plains of flat to gently rolling land, curving ridges of end moraines where the ice front paused, kames, eskers, kettle lakes, and boulders from Canada; glacial grooves are carved into limestone on Kelleys Island; and a former glacial lake left the very flat land of northwestern Ohio, once the Great Black Swamp. South and east of the boundary, in unglaciated Ohio, there is no till: steep hills, narrow valleys and bedrock near the surface. Mastodon and mammoth bones found in Ohio bogs, datable by carbon-14, belong to this history.

Past and present climate change. The rock and ice record shows climate has changed often, usually over thousands to millions of years, driven by changes in Earth's orbit, in plate positions and in greenhouse gases. The same methods applied to modern measurements show that global average surface temperature has risen by more than 1 °C since the late 1800s, and that glaciers worldwide are retreating and sea level is rising. The rock record gives the long baseline against which that pace is compared.

Worked example, a scale model on a field: Lay 4.6 billion years along a 100 m field, so 1 m = 46 million years. The Phanerozoic begins 539 ÷ 46 ≈ 11.7 m from the end; the Mesozoic begins 252 ÷ 46 ≈ 5.5 m from the end; the dinosaurs die out 66 ÷ 46 ≈ 1.4 m from the end; modern humans occupy the last 0.3 ÷ 46 m ≈ 6.5 mm. The first 88 m of the field hold Precambrian time.

Worked example, correlation with index fossils: Outcrop X, in Ohio, shows limestone containing a trilobite species that, in this hypothetical example, lived for only about 2 million years. Outcrop Y, 300 km away in Indiana, shows shale with the same species. Although the rocks differ (limestone and shale, different depths of the same sea), the index fossil shows they formed at about the same time. A third outcrop with brachiopods that lived for 40 million years cannot be correlated as precisely, because its fossils spanned too long a time.

Worked example, reading climate from a column: A hypothetical core from eastern Ohio shows, bottom to top: dolostone with salt layers, black shale with fish fossils, sandstone, coal with fossils of tropical-type tree ferns and scale trees, then unsorted clay with scratched boulders at the surface. Interpretation: an evaporating sea, usually a sign of a hot, dry climate; a deeper, quiet sea with oxygen-poor bottom water; a shoreline or delta; a warm, wet tropical swamp (the plant types show the warmth; coal alone would show only a wet swamp); then, after a long unconformity, glacial ice. Two causes change the climate: the continent drifting through different latitudes and global climate changing. Note that the glacial clay, if the core really came from eastern Ohio, must be from a site north of the glacial boundary.

Worked example, north versus south: A student compares two Ohio counties on a glacial map, one in the northwest and one in the southeast. Prediction for the northwest: flat till plain or lake-plain land, thick glacial deposits over bedrock, few bedrock outcrops, drainage ditches, erratics. For the southeast: steep hills, narrow valleys, thin soil over bedrock, many sandstone and shale outcrops, no till or erratics. The difference follows from whether ice covered the area, not from the bedrock alone.

Investigation, a local field study: Choose a safe, legal site with teacher approval, such as a road cut away from traffic, a quarry that offers school visits, a stream bank or a park with permission to collect loose material. Document the fieldwork in a dated field notebook that lists every step of the investigation as you do it, so another student could repeat it. Plan: (1) record the location with a phone map and photograph the outcrop with a scale; (2) measure and sketch each layer, noting rock type, thickness, grain size, structures and color; (3) record fossils in each layer with a photograph and a sketch, collecting only loose pieces where allowed; (4) compare your layers with the published bedrock map for the area to name the unit and its period; (5) write the history: environment of each layer, whether sea level rose or fell, and any gaps. Result that supports an interpretation: a fossil assemblage and rock type that both indicate the same environment. What the study cannot show: the absolute age, which comes from published radiometric work, and the parts of the record that erosion removed. Wear sturdy shoes, stay away from unstable faces and never enter a quarry without a guide.

3-D local model: Using a published bedrock map and cross-section of your area (for example from the Ohio Geological Survey), build a layered 3-D model from stacked cardboard or foam, one layer per unit, with thicknesses to a stated vertical scale and the glacial deposits on top, and label each unit with its name, period and environment.

Representation task: Build a geologic time scale to scale on a strip of adding-machine paper 4.6 m long (1 m = 1 billion years). Mark the eons and eras, the two mass extinctions named in this lesson, and five Ohio events: Ordovician seas, Silurian salt, Devonian Dunkleosteus, Pennsylvanian coal swamps and the ice ages. Note how crowded the last 0.54 m is.

Independent performance: Write an account of at least 300 words of the geologic history of your county or a named Ohio location, using a published bedrock map and the glacial map. Success criteria: it names the bedrock units, their periods and their environments; it describes at least two fossils found there and what they show; it identifies the unconformity at the base of the glacial deposits and estimates how much time is missing (or, for an unglaciated southeastern location, the gap between the youngest bedrock and the modern soil and stream deposits); it describes the glacial features expected there, north or south of the boundary; and it relates the record to one past climate change.