As wine aficionados, we often indulge in defining soil types of favored vineyard sites by their geologic ages. Thus we have references to Devonian slate in the Mosel, Jurassic marls in Burgundy, Tortonian vs. Helvetian/Serravalian soils in Barolo, Cambrian in Australia’s Heathcote and so on. While these are terms that define geologic age, they have little meaning for viticulture. What interests the grapevine is the soil in which its roots grow. Soil formation is not related to the age of the bedrock from which the soils form. Soils are nearly always far younger than the rocks that they overlie. Still, geologic time terminology is embedded in modern wine marketing jargon, so examining the history of geochronology may give us a better understanding of what these terms mean.
The Geologic Time Scale
Today, few discussions in geology can occur without reference to geologic time and plate tectonics. They are both integral to our way of thinking about the world. It was not always so, however, and we owe much of our understanding of the Earth’s geologic history to a few pioneers of innovative thought in the twentieth century. We can look at a modern geologic time scale and wonder: How was it determined that the Kimmeridgian Age of the late Jurassic Period began 157 million years ago and lasted about 5 million years?
Geology is a relatively young science. While Aristotle, Pliny the Elder and other Greeks and Romans were keen observers of earthly phenomena, the science did not take hold until much later. William Smith, a surveyor, published the first geologic map of Great Britain in 1815. Smith, through his survey work for canals and mines, noted that rocks appeared in the same sequence in areas throughout Britain. Further, he noted that distinctive fossils appeared in the same rocks from location to location. The concept of stratigraphy -- the consistent and sequential presence of horizontal layers of rock over great distances -- became part of geologic understanding.
William Smith’s 1815 Geologic Map
Early geologists became more keenly aware of relative ages of rocks. Logic showed that older rocks were buried under younger beds. The questions about the length of the rock formation process remained. At the end of the 19th century, many geologists thought the earth to be only a few thousands of years old. Biblical scholars had made calculations based on indications found in scripture. Scientists like Charles Darwin, Charles Lyell and physicist Lord Kelvin, however, had all concluded by the end of the 19th century, that the planet must be far older than the biblical calculations implied.
Though they believed the earth was much older than previous generations had thought, the puzzle of how to measure the age of the earth stymied geologists. It was the contemporaneous research of Marie Curie and the discovery of radioactivity that changed the geologic understanding. Subsequent research and discovery by Ernest Rutherford in 1904 showed that radioactive materials decayed at a consistent rate and that radiometric dating of rocks could be a valuable corollary to his findings.
During this period of compelling scientific breakthroughs and discoveries, a brilliant young (born in 1890) geologist, Arthur Holmes, turned his efforts toward developing a geologic time scale. He developed a method using uranium to lead radioactive decay that led him to state that the earth was at least 1.6 billion years old. In 1913, at age 23, Holmes published his book The Age of the Earth that outlined his research and age estimate. This, of course, ignited a great controversy but by the 1920’s his work was widely accepted. Indeed, by that time estimates of planetary age had grown to 3 billion years. Today’s geologists have settled on 4.6 billion years as the likely age of the earth.
Arthur Holmes – Age 22
Arthur Holmes’ influence was as profound in the discussion of continental drift. Geologists had long seen map and stratigraphic evidence that continents now widely separated were once joined. German geophysicist Alfred Wegener had proposed a continental drift theory in 1912, but did not have a mechanism that would explain the motion of entities so vast as continents. Holmes in 1928 postulated that slow convection currents in what is now called the earth’s mantle could provide the motive force necessary for continental movement. It would not be until the 1960’s that studies of mid-ocean ridges and sea-floor spreading provided confirming evidence for Holmes’ theory.
Consider that it has been just over a century that radiometric dating has been used as a geologic tool. It has barely been 50 years since plate tectonics and continental movements based on convective forces has been the accepted norm of geologic knowledge. These were transformative events for the science of geology. Understanding the length of time required for rock formation and subsequent movement and deformation gives us an outlook that was not even contemplated by geologists of the 19th century.
How does this knowledge improve our understanding of wine? Part of the joy of wine is knowing that specific sites consistently yield superior wines. Knowing the geologic forces that created a vineyard site can enhance our understanding and appreciation of the uniqueness of place -- terroir, if you will. For instance, modern understanding of geochronology has allowed us to unravel the story of the dramatic terroir of the Western Cape of South Africa.
Contact Between the Cape Granite and Table Mountain Group
We can observe that the Malmesbury Group Shale is the oldest rock, having formed some 560 million years ago. Ten million years later, while still at great depth, the Malmesbury rocks were intruded by magma -- molten rock that baked the shale along the contact zone and slowly cooled, forming the Cape Granite. During the next 40 million years, continents collided and brought the Malmesbury/Granite combination near the earth’s surface. Then began a long, 150 million year period of deposition of a thick sequence of sands interspersed with clay that formed the Cape Supergroup. Today we can see the contact between the older granite and the flat-lying sandstones of the Table Mountain member of the Cape Supergroup. We also find this sequence in the winegrowing areas of Stellenbosch, Paarl and beyond.
Vineyards in Stellenbosch, South Africa
Vines and vintners know there is a difference between soils derived from the sandstones, shales and granites and those differences are reflected in the resultant wines. The vines, however, probably don’t care that there is a 160 million year or so difference in age among the rocks. For those interested in unraveling the earth’s mysteries, the rock record tells a compelling story. More details will be elaborated as geologic research carries on. So much has been learned in the last century. The next century holds discoveries we have yet to imagine.