As we have seen, variable resistance of bedrock to erosion has been proposed as an explanation for the topography of Manhattan and the Bronx. A similar control of landscape by resistance of bedrock has been proposed for the west bank of the Hudson River. (This control is called "differential erosion.") There, the Palisades is an elongate ridge that parallels the shore of the river. The east side of the ridge is a sheer cliff that rises hundreds of feet. (SEE PICTURE) The west side of the ridge slopes gently downward to the west. The topographic profile X-Y illustrates the asymmetric character of the Palisades ridge and also shows details of the topography of northern Manhattan and the southern Bronx.
1. Print out this page.
2. Examine Figures 3 and 5. Locate profile X-Y on these maps.
3. Refer to profile X-Y and Figures 3 and 5 to answer question 4.
4. The locations of the Hudson and Harlem Rivers are indicated by the blue arrows. Three other important features (indicated by black arrows) are:
The ridge of northern Manhattan _____.
The Palisades ridge _____.
The ridges and valleys of the southern Bronx _____.
5. Rock samples 4 and 5 were taken from the Palisades. What are the names of these rock types?
6. Rock samples 6 and 7 represent the rocks that occur to the west of the Palisades and are also found in the narrow strip of low land between the foot of the Palisades cliff and the waters of the Hudson River. What are the names of these rock types?
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Is sample 6 more or less resistant than samples 4 and 5?____________________
What happened to sample 7 when you tested it?
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Is sample 7 more or less resistant than samples 4 and 5?____________________
8. Does the hypothesis of differential erosion seem to explain the high elevation of the Palisades compared to the areas immediately to the east and west?_______________