When you get to GZ you'll be standing on the crest of a levee that separates Bethel Island from the waters of the Sacramento-San Joaquin Delta. If you look to the east you'll see a channel (Piper Slough), a ridge of reeds, and the vast waters of Franks Tract.
During the last ice age sea level was roughly 300 to 400 feet below current sea levels. At that time what are the shoreline was out beyond the Farallon Islands; and the Golden Gate, San Francisco Bay, Carquinez Straits, and where the delta is were all deep river valleys. With the rise of water levels to roughly modern sea level the area of the delta flooded, became a vast, quiet water body that eventually filled with sediment and peat. Peat is a sedimentary deposit formed almost entirely of plant debris. In a marshy environment, when the plant debris sinks below the water, the low oxygen environment results in this plant debris essentially not breaking down. This peat then can form deposits hundreds (or more?) feet thick.
When the first settlers arrived in California, the delta was a maze of waterways with broad "reedy" island. These islands had a land surface at or even just below the average water level. The soils in these ready island and swamps that surrounded the edges of the delta had soils that consisted of peat and other organic rich soils. This soils did not break down, because being mostly submerged bacteria and other decomposers could not break the organic matter down in the water logged, low oxygen environment. Beginning in the late 1800s farmers began diking off and draining these islands and marginal wetlands to create new farmable acreage. These soils rich in organic matter were extremely productive. The drained soil were no open to infiltration of oxygen from the atmosphere. This cause the organic matter and peaty soils to break down releasing nutrients which would fertilize the crops. However, this break down of the soils had the undesirable effect in that as the organic matter broke down, the soil lost mass as the organic matter was converted to carbon dioxide and released to the atmosphere.*
Now lo these many years later, if you look to the inland portions of Bethel Island and then at the waters of Piper Slough, at the time of reclamation the land surface elevation of Bethel Island was roughly equal to the water level in the slough. Since then the land surface has subsided as aerobic microorganisms (mostly bacteria) have and continue to consume the organic rich soils of the island.
And now a cautionary tale. If you look across the way to Franks Tract, reclamation of this island was done between 1902 and 1906. Farming continued, and the land subsided, until 1937 when a levee failed flooding the tract. The levee was repaired, but another levee failure resulted in the repeated flooding in 1938 after which no more efforts were made to reclaim this tract. Similar levee failures befell such areas as Big Break (1927 and again 1928) and Mildred Island (1983). One interesting note, when they tried to reclaim Big Break after the 1927 levee failure, old sailing ships were filled with soil and sunk to attempt to create a new levee. Some of these sunken sailing ships are still visible, check it out on Google Earth.
As with all Earth Caches you must answer a question (or more) to log this cache. Go ahead and log the cache and send me the answer at the same time. I will only contact you if I feel your answer is inadequate. I'm pretty easy with all good faith efforts, but this is a pretty easy question.
Question: From where you are look at the water level in Piper Slough and look at the land surface elevation in the interior of Bethel Island, below the levee. What is the elevation difference? This is how much land subsidence there has been in the last slightly more than 100 years.
*Organic matter can and does break down in anaerobic (oxygen "free) environments, but at an infinitesimal rate compared to aerobic environments. These peaty islands are maintained by the fact that the rate of accumulation exceeds the rate at which organic matter breaks down anaerobically.
Note, the mosic background shows the relative amount of subsidence in the delta, which exceeds 25 feet in some areas.