LYELL,
CHAPTERS
13-14:
DELTAS,
LACUSTRINE
AND
MARINE
After considering the transporting and destructive action of
rivers and groundwater,
Lyell turns to explore the renovative or reproductive effects of
rivers manifest in deltas. In chapter 13 Lyell examines
lacustrine deltas deposited where rivers enter lakes
and inland seas, and in chapter 14 he describes the marine
deltas which form with the cooperative effect of tides
and currents.
Deltas, which are triangular or fan-shaped, take
their name from the capital Greek letter delta. The Nile river has a
classic delta shape, although many river deltas--such as the
Mississippi--do not. Many slow-moving distributaries
run from the river to the sea, creating the fan-like pattern of
deposition.
Deltas of Lakes
-
- Lake Geneva (221-225)
- Also known as Lac Leman, in Switzerland, about 37 miles
long and from 2 to 8 miles wide.
- At the north end, where the lake is fed by the Rhone River,
the water is turbid and the lake is shallow. At the south end
it is clear and transparent.
- An ancient Roman town situated at the north end of the lake
is now about 1.5 miles inland as a result of about 800 years of
deposition. The strata formed in this area, Lyell argues, would
be nearly horizontal over a length of almost 2 miles and about
600 to 900 feet thick. Coarser deposits from summer months,
when melting snows provide greater energy to the transporting
capacity of the river, alternate with finer deposits from
winter months, when the lake level falls by about 6 feet.
- How long will it take for the entire lake to be converted
into dry land? Present-day experiments and observations might
make such a calculation feasible, but why does Lyell
argue that "it would be more easy to calculate the future than
the former progress of the delta" (p. 223-224)?
- What evidence might be observed if above the present
location of Lake Geneva there once was a chain of upper lakes,
now filled up and obliterated by earlier river deposits?
- Lyell critiques the chronological calculations of Kirwan
and De Luc, two theorists of the earth who had argued against
Hutton, on pp. 224 and 301. What is the basic principle of
geological reasoning that Lyell advances as a remedy to their
"vague and visionary" speculations?
- Lake Superior, (225-227)
- Lyell believes that Lake Superior is the largest fresh
water lake in the world, about 360 by 140 miles and up to 200
fathoms deep.
- What evidence does Lyell cite for inferring that the water
level of the lake was once much higher than it is now? Might
other hypotheses, such as storms or earthquakes (Charles Darwin
had observed the latter to elevate the Pacific shoreline of
Chile), account for the same evidence? How does Lyell eliminate
such alternative hypotheses in this case?
Deltas of Inland Seas
-
- Deltas of the Baltic, 227-232.
- What was the "animated controversy" of the 18th century
recounted by Lyell involving Anders Celsius?
- What evidence was collected as a result of this controversy
(try to identify and make a list of relevant data)? What pains
were taken to obtain reliable and decisive observations during
this time?
- Did the evidence prove that there had been a "diminution of
the ocean" as opposed to an elevation of the land due to
deposition of fresh sediment or earthquakes or some other
cause? What is Lyell's view of the phenomena involved and the
causes in operation which best account for them? What is the
"certainly very unexpected fact" which Lyell infers?
- Delta of the Rhone, 232-235.
- What is the role of "antiquarian monuments" in the
reconstruction of the geological history of the Rhone
delta?
- Describe the physical and biological conditions of "etangs"
(lagoons), located on the landward side of sand bars or barrier
islands.
- How do beds of drifted marine shells become interstratified
with fresh-water strata at the mouths of rivers?
- What processes acount for the chemical precipitation of
solid deposits within the area of a delta?
- Delta of the Po, 235-238.
- What has caused the rate of encroachment of new land upon
the Adriatic sea to accelerate? See the chapters on rivers
for an explanation.
- How far has the water receded from the land in this area
since the time of Augustus?
- Delta of the Nile, 238-239.
- The ancient Hellenistic lighthouse, one of the seven
ancient wonders of the world, was located on the island of
Pharos. Of what use are antiquarian and historical references
for reconstructing the geology of the Nile delta?
- The ancient Greek historian Herodotos said that Egypt was
the "gift of the Nile." What did he mean by this? How did he
explain the origin of the fertile Nile valley?
- What agency now prevents the further extension of the Nile
delta into the Mediterranean Sea? Lyell will explore these
causes in chapters 15, 16, and 17.
Oceanic Deltas
Oceanic deltas are those in which rivers on
entering the sea are exposed to the influence of currents and
tides. Estuaries are "negative deltas" where the
ocean enters the river's mouth and penetrates into the land beyond
the general coastline.
-
- Delta of the Ganges, 240-245.
- Description: The Ganges descends from the Himalayas. The
area of the delta of Ganges is more than double that of the
Nile, almost equal in area to Wales, with the head of the delta
about 220 miles inland. The part of the delta bordering on the
sea is a labyrinth of rivers and creeks, all of which are salt,
but during flood season the Ganges almost assumes the character
of a river entering a lake or inland sea.
- During which season does deposition in the delta prevail
over tidal erosion and destruction?
- Are the strata deposited by the Ganges inclined gradually
or steeply?
- What causes islands to form in the Ganges river?
- How do the examples of the Garial and crocodile suggest
that fossilization is selective, and that not all organisms in
a region are equally-well preserved?
- What happens when high tide is joined by gale-force winds
in the flood season? Reflect upon how Lyell's rhetoric in
discussing this point reconciles words such as "catastrophes,"
"calamities," "havoc," "general order and tranquility," and
"always being governed by the same laws," in order to browbeat
the "extraordinary energy" invoked in the "sudden revolutions
resorted to by cosmogonists."
- Delta of the Mississippi, 245-246.
- What feature of the tides in the Gulf of Mexico accounts
for the shape of the Mississippi delta?
- Mark Twain's account of the
Mississippi delta humorously discusses the possible length of
the delta in the past. Is the Mississippi delta presently
shortening (as Twain would have it) or becoming more elongated?
What are the causes responsible for this?
- What causes delta strata to alternate in character, such as
the red ocherous earth alternating with bluish clay in the
Mississippi delta?
Quantity of Sediment in River Water,
246-249.
-
- Rennell experimentally determined that the quantity of
sediment daily removed from the land by the Ganges is
approximately equal to how many times the weight of the Great
Pyramid of Egypt? Evaluate his methodology and suggest any
improvements you can think of.
- Which is greater, the quantity of matter added to the land by
Mount Etna in a single great eruption, or the annual discharge of
sediment by the Ganges?
- Given the above, what is the relative significance of the
agency of running water to sculpt the continents?
- Is the greater volume of sediment that is transported to the
sea taken from the tops of ridges and mountains, or from valleys?
Explain your reasoning.
Grouping of Strata in Deltas, 249-255.
General laws of arrangement:
- If a lake is bounded by mountains on one side and lowland on
the other:
- The older strata originated on the mountainous side where
numerous deltas formed.
- The newer strata were deposited in the more central parts
of the basin.
- Which of the two types of strata would be composed of
coarser substances? Which would be more homogenous in color and
mineral composition? Which would be more highly inclined? Which
would be irregular in thickness?
- What would be the effects on the above scenario of tides
and currents, or earthquakes?
- What circumstances contribute to the existence of discernible
differences between deltaic deposits of different seasons or
years? Why are alternating beds the norm? (253-255)
- What famous geologist gets the last word in this chapter?
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