Author of the Month
The empirical experience of our limited lifetimes on this planet, imparts to us a tendency to imagine earth’s environment as static. That is the first and most powerful effect upon perception the modern baseline paradigm imparts to our conception of the past. The danger is not that we might dismiss the occurrence of dramatic changes in climate occurring over vast periods – all rational observers accept this. The danger is that we might fail to adequately consider the ramifications of the process of change itself.
Because, from our limited perspective, our world appears so stable, it is tempting to fragment the past into psychologically digestible still images, often framed by a geological, climatological, or cultural title (such as “Eemian Inter-Glacial,” “Eocene,” or “Iron Age”). These frames of reference are, as we know, artificial; for in fact, each stage relates temporally and causally to its precedents and antecedents. We all know this and we realize that it would be erroneous to consider each stage in isolation; yet, here too, awareness of the danger offers no insurance against its effect.
Underworld addresses an inter-stage change process in Earth’s climate: the transformation between the Ice Age and Holocene worlds. In his discussion of deltas however, Hancock neglects to consider explicitly how this and other similar change processes might have affected the nature of these river silt deposits. Consequently, he may have overlooked something of great significance.
Or possibly not.
In a discussion of Taiwan in on page 635 of Underworld, as cited in the last installment of this series, Hancock writes:
These long-lost coastal plains [between Tiawan and the Chinese Mainland, were] fertile with the silt of the Ancient Yangtze and Yellow rivers.
This statement is actually a bit ambiguous. Is Hancock referring only to the former, static presence of river deposits and deltas in these once dry lands, or is he implying the presence of something much more significant; something unique to the Ice Age earth?
What is easy to forget, in discussions of ocean level change, is that a process of transformation occurs during the intermediary stages between the ocean’s upper and lower limits. The waterline moves from its low point to its high point and then back again. This movement produces results easy to overlook when one considers each stage in isolation.
One such result of movement in the relative sea level is the formation of multiple deltas.
When the waters of a river move, they carry with them particles of dirt and soil, suspended in the flow. So long as the water is fast moving, most of the particles remain suspended – and indeed, more are added as the river progresses downhill from high ground to low ground.
Gravity is the primary force by which these particles are eventually released from the river’s grasp, but in order for gravity to overcome the inertia of the river’s movement, the water must be slowed. So long as the water cascades over steep inclines, it will not release its silt cargo.
Two mechanisms, when encountered by the water’s flow, are capable of slowing a river sufficiently to cause silt deposits to settle: large bodies of water and wide, flat expanses of land.
When a river rolls over flat plains, it widens – becoming something akin to a long and twisting, ever-moving lake. Its slackened pace allows gravity to overcome the inherent energy of the water, releasing trapped particles to the bottom and to the riverbanks over a wide and long expansive area.
Oceans and lakes, on the other hand, bring a river’s currents to a standstill. This immediate resistance instantly releases suspended matter in a localized space at the mouth of the river. There, like a cement truck laying down pavement, the river expunges its rich, mineral runoff. These fertile paving stones are the building blocks of deltas.
As the location of the seashore determines the base of the delta, changes in ocean depth alter the landside altitude at which deltas arise. As the water level rises and falls, new delta sediments are laid down in each successive location, producing a step pattern of highly mineralized sediment, ascending from the low water mark to the high water mark (see side-bar).
Rising water levels submerge delta sediments and lay down new sediments at higher altitudes. Falling water levels abandon ancient (but still fertile) deltas far from shore, deposit new silts at a lower altitude, and re-expose previously submerged delta sediments (for an illustration of this process, refer to the accompanying animation).
Wherever there is ample continental-shelf space, the low water marks (during the glacial maximums) reveal long trains of deltas, former deltas, and delta deposits, stretching from the ancient coastline all the way to the new shore. The larger and flatter is the shelf, the larger and richer is the expanse of ancient deltas that form along the river’s lengthening path to the sea. Conversely, where the shelf is narrow and steep, the expanse of fertile silt deposits is minimal.
All of these deltas, new and abandoned, contain rich reservoirs of fertile soils, constantly irrigated by the flow of fresh water.
Recent evidence suggests that the Eemian Inter-Glacial period may have ended very quickly – perhaps even within a stretch of 400 years or less (thus the animation shown is an oversimplification). Such a rapid end would have probably failed to produce a vast succession of fully formed deltas, as there would have been little time between each lowering of the sea level to produce a complete delta formation at the shoreline. Nevertheless, several moderate retreats punctuated the glacial period itself in a yo-yo’s bounce of incomplete warmings. These climate fluctuations produced the same staggered effect upon seaside silt deposits.
Now, even under static ocean conditions, deltas tend to expand, as the constant inflow from connecting rivers introduces additional silts. The process is such that the inward-directed tip of the common delta triangle will extend further and further away from the ocean as more soil collects. This inland growth has its limits however, for eventually, the delta extends to a point where the turbulence of the inflowing river no longer permits the settling of suspended silts and minerals.
Restrained by the natural incline of the surrounding land, a delta will grow only thus far, and no further.
During the Eemian Inter-Glacial period, delta silts were deposited at a position approximate with modern sea level – zero meters (to perhaps as high as plus 7 meters). For thirty thousand years, these deltas expanded inward to their maximum extent. In only a very few cases was this expansion not grossly limited by the narrowness of the above-water escarpment, which tends to steepen sharply at the ancient Eemian seashore. Thus, the Eemian world allowed for the formation of only the smallest of deltas.
When the Ice Age began, water depth decreased and, in many areas, exposed enormous, flat expanses of virgin continental shelf. The inward-directed tips of the newly formed low water deltas became the recipients of 100,000 years of Ice Age, glacial silt deposits. The flatness of the surrounding plains insured for the new deltas, a potential to achieve gargantuan proportions. Unrestrained, in many cases, these deltas would have grown to consume the entire surrounding plain.
By contrast, all deltas of the modern and historical eras have formed at the high water mark of the Holocene and Eemian interglacial periods. As such, they are virtually buttressed against a continental cliff edge. As much as these tiny, barely significant delta systems have historically served as bread baskets for entire civilizations, their food producing capacity pales in contrast to their cousins of the glacial age of gigantism (see animation for comparison).
Thus, the process of climate change, at the beginning of the Ice Age, was certainly capable of creating enormous fields of rich soils; in some cases, fields as wide as entire countries. Nevertheless, these conditions occurred only where the continental shelf was wide, flat, and shallow.
A region such as Cambay, on the western shore of India, is exactly the kind of place we might expect to produce these conditions. During the Ice Age, delta-deposits may have extended here for hundreds of miles, over land now lying at the bottom of the sea. These lands certainly could have provided humans and animals with abundant food resources, as well as the carrying capacity (in terms of land area) to support a relatively large human population.
The same cannot be said of areas along the eastern shore of India. Here, the sub continent’s steep shelf was unlikely to have sustained the formation of such lengthy and concentrated silt deposits. The land’s sharp incline narrowly constrained the dimensions of what few deltas may have formed along its deep shore.
From the perspective of certain, specific geographical locations then, the Ice Age earth might be characterized as an especially fertile world (perhaps even more fertile than our own!), with the bulk of that fertility concentrated on those few lengthy, low lying plains destined to be awash in sea water a few thousand years hence.
However, the Ice Age freeze may have left our ancestors an even more powerful and tantalizing gift than even this.
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