Track 8: Estuary
Sculptor: Jeanette Harris
For more information and activities related to the tracks, contact
the cache owner.
TO VERIFY YOUR FIND, locate the Thunderbird track using the
given coordinates. BEFORE logging your find, send the following
answers in a separate e-mail to me: 1) Stand at the track and
estimate whether the tide is high or low, 2) look at the track, and
give me the names of at least four of the animals depicted on it,
and 3) tell me where in the park you think you might find each of
those animals. DO NOT include these answers in your on-line
log.
GROUP ANSWERS WILL NOT BE ACCEPTED. EACH CACHER LOGGING A FIND MUST
ALSO SEND THEIR OWN ANSWERS AT THE SAME TIME. If you fail to do
this and I delete your log, you can then send your answers and
re-log the find at your convenience.
The story has been told that when Mudtoes the Sandpiper searched
for a home, he was told that the best place was in King’s
Town on the Carpenter’s land, but that no one went there any
longer because of the great water snake living there. The water
snake’s body was so enormous that its tail was in a lake, its
body coiled through the forest, and its head was in the saltwater
bay. It was so big that is only needed to breathe in and out twice
a day.
Mudtoes battled the snake, and won by grabbing the snakes
tongue, splitting it into a fork as a result, and letting go only
after the snake promised to leave forever and then slithered away.
Soon the surrounding land once again flourished. The trough where
the snake used to be was filled with a mixture of fresh water from
the lake and salt water from the sea. If you return to this spot
today, you will remember the snake as you watch the water continue
to “breathe” in and out twice a day. This type of area,
where salt and fresh water mix, is now referred to as an
ESTUARY.
You may be wondering, “Why does the water continue to go
in and out twice a day if the snake is no longer there?” It
is now a result of gravitational pull from the moon and the sun
causing water motions referred to as “tides.”
Let’s discuss tidal action a bit further.
Tides – the periodic rises and falls of large bodies of
water, caused by the gravitational interaction between the Earth
and the Moon.
The earth and the moon are two great masses that have a
significant gravitational pull on each other. This is what keeps
the moon in orbit around the earth, and it is also what causes
tides to occur in the ocean. Picture the earth with a uniform level
of water all around it. The moon’s gravity pulls on the
earth, and pulls the water towards it. The water moves up into a
slight bulge on the side of the earth that faces the moon.
At the same time, there is a force pulling water out in the
opposite direction of the moon. To understand this force, you need
to picture the earth and the moon as one unit. Picture two unequal
balls on the ends of a stick. If you spin this stick around, you
can imagine the force that a particle might feel if it were on the
far end of either the moon or the earth. It would feel a force
outward, away from the center of the spin. This is called the
centrifugal force. The water on the far end of the earth, away from
the moon is always being pulled out from the center of the spinning
earth-moon unit.
The gravitational and centrifugal forces are constant, always
pulling water towards the moon and directly away from the moon. The
forces in either direction are equal to each other. The bodies of
water that feel these forces change constantly as the earth rotates
within these forces, but the force directions are always toward and
away from the moon.
The tides are caused mainly by the gravitational attraction of
the moon and the earth, but there is also a gravitational
attraction between the earth and the sun. The effect of the sun
upon the tides is not as significant as the moon’s effects.
Basically, the sun’s pull can heighten the moon’s
effects or counteract them, depending on where the moon is in
relation to the sun.
In one month, the moon rotates around the earth. When the moon
is between the sun and the earth (at new moon), the sun’s
gravitational pull is in the same direction as the moon’s.
During these days the high tides are higher and the low tides are
lower than they'd be with just the moon’s pull alone. This is
called spring tide.
The same thing happens when the moon is on the direct opposite
side of the sun (full moon). The two gravitational forces work
together to make high high tides and low low tides.
When the moon is in its first quarter or its last quarter, the
sun’s gravitational pull is in perpendicular direction to
that of the moon. The sun pulls water away from the areas of high
tide to the areas of low tides, resulting in lower high tides and
higher low tides. These are called neap tides.
For an animated display of tidal action, go to:
http://www.sfgate.com/getoutside/1996/jun/tides.html
