Margaret M. Loos
As we stand on the beach at Lighthouse Point, some of the features that might catch our fancy, geologically speaking, are the coarse sands of the man-made beach, the massive granite boulders that form the borders of the beach and front the lighthouse, the actual material that makes up the lighthouse, and the great granite blocks cut to shape a breakwater. The newly poured asphalt of the added parking lots cover much, but not all, of the uneven terrain abundantly blessed with rocks of all sizes. As our gaze sweeps toward the New Haven harbor, the graceful curve of Morris Cove is broken as the modest cliffs of Forbes Bluff jut forward. Looking past the navy and Coast Guard facilities and recreational area of the East Shore Park we see the ribbon of the Quinnipiac River widen into a small, but active, harbor. A point of land eases into a long flat landfill area which supports the Long Wharf commercial enterprises, then a small bit of marsh, a cove and finally the West Haven beaches that reach to Bradley Point.
How did such an environment come to be? How do geologists explain these features? Students will not be surprised to hear that early explanations were based on man’s self-centered view that the earth was a place prepared for man, existing for him alone and therefore, the reshaping of the earth could only be explained by the creator’s reaction to man’s actions—a great flood. However, man is a curious creature who looks for relationships, who wishes to understand his environment in order to control it. During the 1800’s geologists’ extensive observations and collection of information led to development of currently accepted theories about the geologic processes that built and carved the features of our earth and the geophysicists of the twentieth century gave further explanation and documentation.
The processes that shape the earth can be divided into the
constructive
, or uplifting forces, and the
destructive
, or wearing away forces, such as erosion, weathering, and submergence in our theater of study, the New Haven harbor. It is easier on our beach to deal first with the destructive forces which rearrange the earth we walk on. We find them in our field study as small models which can explain some geological phenomena of a much grander scale. For instance, after a heavy rain would be a particularly advantageous time to examine the processes of erosion carried on by moving water. When we consider the cycle of water on the earth we would think of
precipitation
, (snow, sleet, hail or rain, and condensation) the
transport of water
on the earth, and
evaporation
or return of the moisture to the atmosphere. This is the hydrologic cycle.
1
The transport stage is the most active in shaping the earth, principally in the process of erosion where material is transported by moving water. As the water runs off the higher shore rocks, down through layers of different sized rocks and sand particles, the heavier rocks remain solidly in place and the water is forced to flow in a different direction, perhaps carrying off a few particles of the rock. Smaller rocks are carried a distance, perhaps to become lodged against the larger. Some grains of sand are rearranged, some carried relatively long distances to be dropped off, and the finer particles are carried in suspension, or actually dissolved, losing their particulate character. The result is a rearrangement of the small strip of beach to form a small valley or valleys. We can identify some of the contributing factors as the resistance of the solid materials, the strength and magnitude of the flow of the stream, the gradation of the land over which it flows, and the physical and chemical makeup of the material the stream encounters. Sometimes we can determine the origin of the material by examining it. In addition the stream may deposit the material (
alluvium
) in certain formations, such as a triangle or delta. The stream may curve or form
meanders
, or be cut off in a series of short, or braided streams, or cut off as pools called
oxbows
.
2
We may use this model to explain the action of rivers in general. They also must have comparable abilities to transport and deposit
s
ediments. The diagrams on the following page illustrate some of the possible formations in river or stream valleys.
(figure available in print form)
(1) Original Valley
(figure available in print form)
(2) Branching
(figure available in print form)
(3) Braiding
(figure available in print form)
(4) Meander
(figure available in print form)
(5) Oxbow New Course
STAGES OF DELTA DEVELOPMENT
Symbols
Sediments
(figure available in print form)
Marshes
(figure available in print form)
(1) What changes occur in the sediments and marshes at each stage?
(figure available in print form)
(2) How would changes in amounts of precipitation and evaporation affect this process?
(figure available in print form)
(3) Examine some maps to find some delta formation in an atlas. Find the Mississippi delta.
(figure available in print form)
Another agent of erosion on the beach (and other environments) is wind. When the sand surface is dry and a strong wind is blowing the sand grains seem to leap quickly for short distances and to form a light film moving across the beach. This is called saltation. The sand surface seems to creep along. Although this is more common with finer sands, it can be discussed on the beach and sands from other beaches (including black sand) can be displayed. Sometimes enough sand will accumulate in a spot as a small mound and will start rolling down hill making a smooth slipface or a tiny sand dune. We may surmise that these processes to change the features of a beach are determined by the strength of the wind and the size of the particles and the type of terrain.
Students may learn (either on the field trip or in the classroom) that the agents of transportation in erosion are:
-
1. Water in the forms of rivers, waves or currents.
-
2. Gravity, from small slippage to avalanches.
-
3. Wind, from the movement of one grain of clay to massive dust storms.
5
Since the size of the sediment particles is an important factor a gradation of these sizes gives an opportunity to learn measurements ranging from those discernible and measurable on a metric ruler to microscopic particles. They are, gravel—greater than 2mm, sand—from 1/16mm to 2mm, silt—from 1/256mm to 1/16mm, and clay particles—less than 1/256mm.
6-
The processes we have observed or discussed in erosion are:
-
1. Saltation, on the beach
-
2. Suspension of materials in moving water
-
3. Solution, dissolved material in moving water
-
4. Traction , dragging or pushing of material by moving water
In examining the beach we may find ripple marks, burrowing, different shaped deposits and beds, and maybe, in perfect conditions, mud cracks. They should aIl be discussed for future reference. Animal and plant life give opportunities for material for side projects and to give evidence that their remains must contribute organic material to the sediments. The salt present in the water of the sound also makes an addition to the sediments.