Hold on to your hat. Fort Funston features 200-foot high sandy bluffs on San Francisco’s southwest coast where the winds blow reliably wildly!
Fort Funston is on a bluff made up of sedimentary rocks of the Merced Formation. The Merced Formation was deposited in a small sedimentary basin that formed along the San Andreas fault during the last two million years.
Ocean breezes collide with the bluff, and in their effort to get over the bluff provide support for hang gliders and birds that effortlessly glide along the upwelling currents. Although the bluff may appear to have been here forever, in geologic terms it has been here only an instant.
Marine Terraces
The flat surface upon which the Viewing Platform, parking area, and the remainder of Fort Funston are situated is called a marine terrace.
This terrace, which is now 150 feet above sea level, was cut by wave action during the Sangamon interglacial period, about 100,000 years ago, when sea level was higher than it is now.
As the climate became cooler during the Wisconsin glacial period, glaciers formed, sea level fell, and the shoreline retreated to the west of Fort Funston. The land along the shoreline was also uplifted slowly, a few tens of feet. By the end of the Wisconsin, about 10,000 years ago, the shoreline had retreated westward ten miles or more. As you stand on the Viewing Platform, imagine that there is no bluff and that the terrace you are standing on extended so far to the west that you cannot even see the Pacific Ocean.
As the Wisconsin glaciers began to melt during the last 10,000 years, sea level rose and the ocean began cutting a new wave-cut platform into the rocks along the shoreline at a lower level. The old wave-cut platform was separated from the new platform by a sea cliff, or bluff. As the new wave-cut platform cut further east, the bluff retreated east an inch or so a
year until it arrived at its present position. The bluff is still being cut eastward, and in a few tens of years will claim the Viewing Platform and then Highway 1 sometime thereafter. The remaining part of the old wave-cut platform is now preserved as a marine terrace.
Marine terraces are common along the California coast north and south of San Francisco. The terraces were preserved because most of the California coastline has been slowly uplifted several hundred feet over the last million years. In general, the highest terraces are the oldest and were cut during the early interglacial stages, and the lowest terraces were cut during the late interglacial stages. Because there have been many fluctuations of sea level during the glacial period, and because the amount of uplift has varied along the coast, it is difficult to correlate specific terraces over long distances. There are no terraces between Fort Funston through the Marin Headlands. This area subsided during the Pleistocene. Indeed, there would be no San Francisco Bay if this area had been uplifted as were the areas to the north and south.
Bluff
The bluff at Fort Funston is made up of sedimentary rocks that were deposited in a small sedimentary basin that had formed along the San Andreas fault during Pleistocene time. These sediments, called the Merced Formation, are exposed as distinct layers that can be seen along the bluff from Lake Merced southward to the San Andreas fault near Mussel Rock, a distance of about four miles. Some of the layers of sedimentary rocks are nearly horizontal and some are tilted to the north, indicating that the rocks have been uplifted and folded in the few hundred thousand years since they were deposited.
To log this earthcache, please answer the following questions and send me the answers. You may log immediately after answers sent, I'll contact you in case something is wrong.
1. Fort Funstion Coastline
17000 years ago, you wouldn't have been at she shoreline. It was 26 miles west, past the Farallon Islands. Estimating a continious progress, what do you think, where will the shoreline be 17000 years in the future? Describe with your own words why!
2. Mussel Rock
Look south and you will see Mussel Rock. What do you think, how far offshore is Mussel Rock? Estimate its size!
3. Hang Gliders!
Watch the windsock. What was the wind speed and direction during your visit?
4. Take a Picture (Optional)
Take a picture of yourself (or your GPS, if you prefer) and attach it to your log. Please make sure not to spoiler any answers.