Map Interpretation
ACTIVE-MARGI N COASTLI N ES
Point Reyes National Seashore lies north of San Francisco on the
rugged California coast. Point Reyes consists of resistant bedrock that is being eroded by the forces of the sea. Marine life is
abundant, including many birds and marine mammals (note Sea
Lion Cove below Point Reyes). A hilly area, Punta de Los Reyes,
separates Drake’s Bay and Point Reyes from Tomales Bay (visible
on the aerial photograph as a ribbon of water across the top, but
barely showing in the northeastern corner of the map).
The oblique aerial photo of Point Reyes was taken from a highaltitude NASA aircraft. Color infrared film makes vegetation appear
red. The view is looking northeast. Trending across the upper portion
of the photograph is the San Andreas Fault, which forms the linear
Tomales Bay. The San Andreas Fault also separates the Pacific plate,
on which Point Reyes is located, from the North American plate,
which underlies the area east of the fault. Point Reyes is moving
northwest along this fault. Emergent coastal features and recent tectonic activity characterize active-margin coastlines.
Point Reyes has a Mediterranean climate, influenced by the
cool offshore California current. Here, the sea is not hospitable for
swimming because of the uncomfortably cool seawater, high surf,
and dangerous rip currents. The coastal location and onshore westerly winds create a truly temperate climate. Although very hot and
very cold temperatures are rarely experienced, this is one of the
windiest and foggiest coastlines in the United States. It is an area of
rugged natural beauty with wind-sculpted trees, grasslands, rocky
sea cliffs, and long sandy beaches.
Interpreting the Map
NASA
1. Which area of the coast is most exposed to wave erosion? What
features indicate this type of high-energy activity?
2. Which area of the map is under the influence of strong longshore currents? What is the general direction of flow, and what
coastal feature would indicate this flow?
3. Looking into the future, what may happen to Drakes Estero?
What would Limantour Spit become?
4. Which area of the map appears to have strong wind activity?
What would indicate this? What do you think the prevailing wind
direction is?
5. Locate examples of the following coastal erosional landforms:
a. Headland
b. Sea stack
c. Sea cliffs
High-altitude oblique aerial photo of Point Reyes, California.
6. Note that the bays and esteros (Spanish for “estuary”) have
mud bottoms. Because the creeks and streams are very small
in the region, what is the probable source and cause for movement of the mud?
7. Note the offshore bathymetric contours (blue isolines).
Which has a steeper gradient, the Pacific coast (west side) or
Drakes Bay (east side)? Why do you think there is such a
great difference?
Opposite:
Point Reyes, California
Scale: 1:62,500
Contour interval = 80 ft
U.S. Geological Survey
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