THE ORIGIN OF NYC LANDFORMS
~EVALUATING AND COMPARING HYPOTHESES ~
You have investigated three hypotheses as to the origin of major landforms of the NYC region:
  • The Hypothesis of Differential Erosion
  • The Hypothesis of Deposition
  • The Hypothesis of Deformation.
You have tested each of these hypotheses by making predictions, assembling and 'playing' with data, and evaluating the predictions. Now, for each landform, you need to evaluate and compare the hypotheses and come to justified conclusions as to which hypotheses are most likely true.

It is important to note that it is quite acceptable for you to conclude that, for a given N.Y.C. landform, (a) more than one hypothesis has good evidence to support it and (b) that you have no way to choose between them, given the available data.

For example, the gently sloping plain in southern Brooklyn is underlain by loose, well-sorted layers of sediment that are inclined gently towards the south. Moreover, the layers are parallel to the surface of the land.

Thus, the gently sloping plain in southern Brooklyn satisfies the tests applied for both the hypothesis of deposition and the hypothesis of deformation, as explained below.

  • Support for Deposition:
    Deposition is supported by the loose character of the sediment. The good sorting and layering supports the action of flowing water (glacial meltwater streams or ordinary stream flooding). Since streams flow down inclined surfaces, the deposits themselves may form inclined layers (see drawing to right). The inclination of the layers does not necessarily indicate that the layers have been tilted. The top of the topmost layer may form the surface of the land; unless the top layer has been unevenly eroded, the layering and the land surface may be parallel.
DEPOSITION OF INCLINED LAYERS

  • Support for Deformation
    Deformation is supported by the presence of inclined layers that are parallel to the surface of the land. The layers may have been deposited horizontally or at a lower inclination than they now display. Their present inclination is due to tilting of the layers subsequent to their deposition.
TILTING OF DEPOSITED LAYERS

Geologists are commonly faced with the dilemma of several seemingly equally well-supported hypotheses. In this case, to decide between the hypotheses of deposition and deformation, additional tests (predictions) need to be devised. To do so requires a knowledge of processes and materials beyond the scope of this course.

What is important for you:

  • It is NOT of prime importance that you make a 'correct' choice amongst two or more seemingly equally strong hypotheses.
  • It is important that you recognize when more than one hypothesis for the origin of a particular landform is supported by the tests performed .
  • It is important that you be able to justify your evaluation of hypotheses proposed for the origin of each landform.

WHAT YOU NEED TO DO:

  1. Click on the blue button and go to the Hypothesis Evaluation and Comparison Table. Follow the instructions and record your answers on worksheet H-6. Make sure to:
    • Look at the Worked-Out Example
    • Use the Answer Checking device.
       

  2. DRAW CONCLUSIONS:
    • For a given landform, if there is only one hypothesis that merits a "Likely" evaluation, and the other hypotheses merit "Unlikely" evaluations, then the origin of that landform may be considered uncontroversial.
    • In all other cases, the origin of the landform may be considered open to question.

  3. Indicate your conclusions concerning the origin of the four NYC landforms.

    1. Type the phrase "uncontroversial" or "open to question" after the name of the landform.
    2. After each hypothesis of origin, place the letter (L) for Likely, (U) for Unlikely, or (NE) for No Evidence.
    3. The first landform has been done for you. Since only one hypothesis of origin is considered 'likely', the mode of origin has been designated 'uncontroversial'.
WHAT GEOLOGISTS HAVE CONCLUDED

After completing your evaluation and comparison of hypotheses, you will have found that on the basis of the information provided, the origin of two of the four NYC landforms under discussion remains "open to question". Geologists, however, in response to a more extensive data base than is provided in these exercises, consider the origin of all of the four NYC landforms under discussion "uncontroversial". How they have decided that the two "open to question" landforms are, in their eyes, "uncontroversial", and which hypotheses they have accepted, is considered in "THE LANDFORMS OF NEW YORK CITY - PART V".

©2003
David J. Leveson