In the 17th century, Archbishop James Ussher, the Anglican primate of all Ireland declared according to the Bible that the Earth had come into existence on Sunday, Oct. 23, 4004 BCE.
This meant that our planet was less than 6,000 years old. For several centuries, this date was added as a footnote to Genesis.
In this same century, the Danish scientist Nicholas Steno (later a bishop) developed the principles of stratigraphy or strata in a book.
These were in layered rocks, younger strata are above older strata, strata in layered rocks were originally formed horizontally, layers are continuous across long distances, and layers that cut across other layers were younger.
But it wasn’t for several more centuries before the ages of the layers and the Earth itself could be determined.
Scottish geologist and naturalist James Hutton (1726-1797) regarded that abrupt changes in layered rocks indicated large gaps in time during which the older layers were eroded, before the new layers were deposited.
These gaps in time were known as unconformities. Hutton also realized that beds of layered rocks, originally horizontal could be thrust upward, tilted and eroded over vast time spans.
These spans might have been caused by the advances and retreats of seas and oceans. Hutton originated the concept of “deep time,” covering millions or even billions of years.
The concept of a geologic map was developed by the English geologist William Smith (1769-1839), who as a surveyor became very familiar with the topography of England, Wales and Scotland through his work on coal mines.
Smith felt that the occurrence of the same kinds of fossils within widely separated layers might indicate that the layers were of the same age. Smith was self-taught and his maps were largely ignored by academics. But decades later, he became known as “the father of English geology.”
Another pioneer was the Scottish geologist, Charles Lyell (1797-1875). Lyell put the insights of Steno and Hutton into his 1830 book, “The Principles of Geology.”
Lyell is famed for advocating uniformitarianism — the idea that the processes now acting on the Earth acted in the past and will continue to act in the future. Lyell’s ideas were strongly opposed by the catastrophists who held that great changes in the Earth were due to violent and sudden events.
Modern geologists accept that both slow, continuous changes and sudden upheavals play a role in Earth’s history.
The Swiss-American geologist Louis Agassiz (1807-1873) established that most of the northern hemisphere had been sheathed in great thicknesses of ice.
Agassiz and his fellow geologists found evidence of five major ice ages, each with many advances and retreats of glaciers over a period of 2.5 billion years.
In the early 1900’s, Alfred Wegener, a German scientist proposed that the continents had drifted apart in the distant past.
He noted how the coastlines of South America and central Africa and the coastlines of Eastern North America and northern Africa matched up. This correspondence also applied to fossils and mountain ranges.
But Wegener couldn’t provide a mechanism to cause this motion. In the 1960s, magnetic stripes were found on the rocks on the ocean floor, confirming the drift.
Today this last merger of continents is known as Pangea (all lands). The Earth’s crust is divided into major plates that creep apart, collide (subduct) and scrap by each other, causing Earthquakes and chains of volcanoes.
In the 1940s, Serbian astronomer and climatologist Milutin Milankovic discovered that variations in the Earth’s orbit about the sun and the tilt of the Earth’s axis influenced the amount of solar heating of the Earth’s surface.
This effect, called the Milankovic cycle, can explain the most recent ice ages (called the Quaternary glaciation), with last episode ending 12,000 years ago.
SKIES IN THE WEEK AHEAD: Dawn begins about 5:50 a.m., sunrise is about 6:52 a.m., the sun peaks at 1:12 p.m., sunset is about 7:32 p.m. and dusk ends about 8:30 p.m. Daily sunlight lasts 12 hours and 40 minutes, with the sunlight diminishing about two minutes each day.
The moon is full on Friday. This is the Harvest Moon, offering early evening moonlight for the coming weekend. The bright planet Jupiter is dropping out of sight in the southwest about 10 p.m.
Bob Doyle, professor emeritus at Frostburg State University, invites any readers comments and questions. E-mail him at email@example.com. He is available as a speaker on his column topics.