Capturing Time Dorothy Merritts THE HARRY W. & MARY B. HUFFNAGLE PROFESSOR OF GEOSCIENCES DEPARTMENT OF EARTH & ENVIRONMENT FRANKLIN & MARSHALL COLLEGE Robert C. Walter THE DR. EARL D. STAGE & MARY E. STAGE PROFESSOR OF GEOSCIENCES CHAIR, DEPARTMENT OF EARTH & ENVIRONMENT FRANKLIN & MARSHALL COLLEGE Without the vibrant layers of sediments and rocks that draw the attention of painters and geologists alike, a record of deep time and our understanding of it would not exist. Deciphering millions to billions of years of Earth’s history from rocks, mountains, and landscapes seems unfathom- able. Yet, deep time has captivated scholars for centuries, particularly geologists who strive to reconstruct Earth’s history from the rock record. Tony Foster expresses his intellectual fascination with this subject through his artistry. Those trained as geologists view the present as a vital key to understanding Earth’s past. This perspective is grounded in the principle of uniformitarianism, in which it is logically understood that processes occurring on Earth today are comparable to those in the past. For example, the physical laws (such as the law of gravitation and Newton’s laws of motion) that govern astronomical and geological processes remain unchanged. Thus, layers of sand in ancient sandstone rocks may have been deposited hundreds of millions of years ago along a beach by waves and fluctuating tides, processes still observable today. Studying these modern processes might enable a scientist to interpret, with some certainty, the ancient environment in which that sandstone formed. Geologists did not know the age of the Earth or when various geological events took place until the mid-20th century. Before then, it was unknown that rocks found deep within the Grand Canyon were over a billion years old. Nor was it known that the canyon cutting is much younger, having been carved in the last few tens of millions of years. In fact, during the 17th century, religious scholars in Europe believed that the Earth was only about 6,000 years old. In 1664, for example, Archbishop Ussher of Ireland claimed that the Earth was created precisely on October 23, 4004 BC, by tracing genealogical evidence from the Bible. As scientific methods for determining rock ages were developed in the last century, mainly through radioactivity to measure geologic time, the estimated age of the Earth increased significantly. Today, scientific evidence reveals that Earth is around 4.6 billion years old, roughly 700,000 times greater than the age estimate from the mid-1600s. This revelation caused a radical shift in human perception of time, but several earlier thinkers were well ahead in their understanding of deep time. Leonardo da Vinci (1452–1519), the Italian artist, scientist, and engineer, stands out for his advanced ideas regarding the age of the Earth. Fascinated by Earth’s materials, da Vinci studied layers of sand, gravel, and fossil debris on the mountainsides of the Apennines in Italy. High in these mountains, he noticed that the fossilized shells and other skeletal remnants resembled clams, crabs, and oysters on modern beaches. Since the time of Aristotle (384–322 BC), such fossils have been interpreted in various ways: as shapes that developed within the rock and merely hap- pened to look like living creatures, as Creation’s mistakes that never became living, or as the remains of victims of the biblical great Flood (Noah’s flood). During Leonardo’s time, the Earth’s surface was believed to be stable and unchang- ing. It was thought to have formed during Creation several thousand years ago and been modified only once thereaf- ter, during the Flood. Leonardo’s contemporaries could not deny that the fossils garnering his attention resembled seashells found along the coast, leading some to suggest that the shells were transported to the mountainsides during the Flood. However, Leonardo disagreed. He noted that the assem- blages of fossils were often wholly intact and that the shells of clams and other relatively immobile organisms seemed to be in their “growth positions.” He concluded they had not been moved to their present locations in the Apennine Mountains by a flood. Furthermore, based on his observa- tions of live clams, he argued that the fossilized ones could not have independently moved to the mountainsides during the Flood. Clams dig narrow furrows in which they lift and drop themselves, moving only about a meter each day. They would have needed far longer than the 40-day duration of the biblical Flood to cover over 100 km (185 miles) from the coast, especially while ascending a steep mountainside. Leonardo instead proposed that a sea once covered the land, and the land might have been raised since that time. Noting the occurrence of earthquakes and ground movement in the area today, he suggested that they might be linked to the uplift of the rock. In other words, each earthquake could raise the seafloor a few meters, and over many millions of years, marine organisms and surrounding sediments could be raised hundreds to thousands of meters above their original position. In all his deductions, Leonardo was correct, but few people were aware of his ideas until his journals were made public some 200 years after his death. Many of his ideas presaged the modern theory of plate tectonics, developed in the mid-20th century. Reflections on Time by Invited Scientists, Scholars and Friends GEOLOGICAL TIME 96
