Think there's no way to rid yourself of the bitterness you feel toward others over past transgressions? Think again.
Part of every continent gets an occasional wash over by the ocean. Right now, Hudson Bay and the submerged continental shelves of North America are part of the sea’s inundation of the land. By comparison with the open, or deep, ocean, such seas are shallow and are called epicontinental (epi = on) or epeiric (epeiros = mainland) seas. The seas are also called transgressions, and they have several causes, including the melting of glaciers whose waters run off the land to increase the volume of seawater. Transgressions also result from the subsidence of the land in response to tectonic processes, such as seafloor spreading and the ripping apart of continents. The latter is what’s going on right now in the Great Rift Valley in eastern Africa and the Basin and Range system in Nevada, both sites of future epeiric seas.
Transgressions of the sea are generally slow in human terms, taking not just a lifetime, but thousands to millions of years.* So, don’t get out your scuba gear or life vest because you won’t be sailing out of the Port of Las Vegas any time soon. For geologists, however, the formation of epicontinental seas can occur relatively rapidly against the backdrop of land more than a billion years old. Typically, the more rapid transgressions occur as glacial ice melts during interglacial warming periods like the current one that began about ten millennia ago. But there’s a back-and-forth cycle here: The opposite process, called regression, can be relatively rapid as climates turn cold and snow accumulates to form glaciers. Locking up Earth’s water in ice on the land causes sea level to fall without a balancing return of water through river runoff. You could also think of the Mississippi Delta as a regressive area because just 8,000 years ago, much of it didn’t exist. Sediments from the mighty river and its distributary rivers filled in areas adjacent to the coast, making new land.
Of course, the transgressive-regressive cycle is a bit more complex than the above explanation, but there is a point to be made here. The processes of ocean water covering and uncovering the land leaves behind a sequence of sediments that geologists use to identify periods of inundation and drying. Typically, as the sea covers the land, its leading water line is marked by beach materials with which we are all familiar: Sands, some coarse, some medium, and some fine. As the sands migrate inland, they are followed by silts, and they, in turn are followed by muds (clays) as deeper and deeper water migrates over the land. Given sufficient time and deposition, a transgressive sequence is one of muds overlying silts overlying sands. The differences in the particle sizes reflect the differences in turbulence, with nearshore water having a violence not associated with quiet deeper water, where tiny mud particles can settle. When the sea leaves the land, the sequence of sediments reverses, with sands overlying silts overlying muds. Over long periods, the sequences can reverse many times, building thick sequences of sediments that can turn into sedimentary rocks.
So, what’s the point I hinted at? In a way, the transition of deposits from transgressions and regressions reflect our own past with others. Our personal histories are complexes of transgressions and regressions, mostly minor, but some major. At times we merely lap over the edge of another’s life like water over the continental shelf. At other times, we are like widespread epeiric seas. And every one of us is also like a continent. Just as we have intruded on the lives of others, so others have intruded on ours. That interpersonal history has left a sequence of transgressive and regressive sediments.
We know the causes of ocean transgressions and regressions from the work of geologists. We have a bit more trouble determining the causes of our own transgressive and regressive sequences. Maybe we should take a hint from geological studies first to understand the interlayering of sequences in our lives and second to identify our current phase. Are we inundating another’s “land,” or are we retreating. In either case, we are leaving lasting deposits on personal history because, just as geologists can read the history of transgressions and regressions from the sedimentary deposits, so can read personal histories by what remains from the fluctuations in interpersonal relationships.
We live dynamic lives on a dynamic planet. So, both kinds of sequences can be eroded as the land rises in a mountain-building episode. With increased elevation comes increased erosion of rocks once laid down as the sediments of fluctuating seas. Raising those products to higher elevations can flush their products out to sea. Like the changing surface of a continent, humans can also change. It just takes a bit of uplift to wash away the products of old transgressions.
*You can, if you’re interested, research ancient epicontinental seas like the Sauk, Tippecanoe, Kaskaskia, Absaroka, and Tejas.
Part of every continent gets an occasional wash over by the ocean. Right now, Hudson Bay and the submerged continental shelves of North America are part of the sea’s inundation of the land. By comparison with the open, or deep, ocean, such seas are shallow and are called epicontinental (epi = on) or epeiric (epeiros = mainland) seas. The seas are also called transgressions, and they have several causes, including the melting of glaciers whose waters run off the land to increase the volume of seawater. Transgressions also result from the subsidence of the land in response to tectonic processes, such as seafloor spreading and the ripping apart of continents. The latter is what’s going on right now in the Great Rift Valley in eastern Africa and the Basin and Range system in Nevada, both sites of future epeiric seas.
Transgressions of the sea are generally slow in human terms, taking not just a lifetime, but thousands to millions of years.* So, don’t get out your scuba gear or life vest because you won’t be sailing out of the Port of Las Vegas any time soon. For geologists, however, the formation of epicontinental seas can occur relatively rapidly against the backdrop of land more than a billion years old. Typically, the more rapid transgressions occur as glacial ice melts during interglacial warming periods like the current one that began about ten millennia ago. But there’s a back-and-forth cycle here: The opposite process, called regression, can be relatively rapid as climates turn cold and snow accumulates to form glaciers. Locking up Earth’s water in ice on the land causes sea level to fall without a balancing return of water through river runoff. You could also think of the Mississippi Delta as a regressive area because just 8,000 years ago, much of it didn’t exist. Sediments from the mighty river and its distributary rivers filled in areas adjacent to the coast, making new land.
Of course, the transgressive-regressive cycle is a bit more complex than the above explanation, but there is a point to be made here. The processes of ocean water covering and uncovering the land leaves behind a sequence of sediments that geologists use to identify periods of inundation and drying. Typically, as the sea covers the land, its leading water line is marked by beach materials with which we are all familiar: Sands, some coarse, some medium, and some fine. As the sands migrate inland, they are followed by silts, and they, in turn are followed by muds (clays) as deeper and deeper water migrates over the land. Given sufficient time and deposition, a transgressive sequence is one of muds overlying silts overlying sands. The differences in the particle sizes reflect the differences in turbulence, with nearshore water having a violence not associated with quiet deeper water, where tiny mud particles can settle. When the sea leaves the land, the sequence of sediments reverses, with sands overlying silts overlying muds. Over long periods, the sequences can reverse many times, building thick sequences of sediments that can turn into sedimentary rocks.
So, what’s the point I hinted at? In a way, the transition of deposits from transgressions and regressions reflect our own past with others. Our personal histories are complexes of transgressions and regressions, mostly minor, but some major. At times we merely lap over the edge of another’s life like water over the continental shelf. At other times, we are like widespread epeiric seas. And every one of us is also like a continent. Just as we have intruded on the lives of others, so others have intruded on ours. That interpersonal history has left a sequence of transgressive and regressive sediments.
We know the causes of ocean transgressions and regressions from the work of geologists. We have a bit more trouble determining the causes of our own transgressive and regressive sequences. Maybe we should take a hint from geological studies first to understand the interlayering of sequences in our lives and second to identify our current phase. Are we inundating another’s “land,” or are we retreating. In either case, we are leaving lasting deposits on personal history because, just as geologists can read the history of transgressions and regressions from the sedimentary deposits, so can read personal histories by what remains from the fluctuations in interpersonal relationships.
We live dynamic lives on a dynamic planet. So, both kinds of sequences can be eroded as the land rises in a mountain-building episode. With increased elevation comes increased erosion of rocks once laid down as the sediments of fluctuating seas. Raising those products to higher elevations can flush their products out to sea. Like the changing surface of a continent, humans can also change. It just takes a bit of uplift to wash away the products of old transgressions.
*You can, if you’re interested, research ancient epicontinental seas like the Sauk, Tippecanoe, Kaskaskia, Absaroka, and Tejas.
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