I was walking with a forest ranger through the Headwaters Forest near Fortuna California on 9/16/07. We noticed large numbers of large 6 inch plus sea shells on a recently eroded old logging road . We were approximately 100 to 150ft + over sea level maybe 7 to 10 miles from the Pacific ocean.
How long can sea shells last ?
There is no single or simple answer to this question. The length of time that a sea shell can last depends upon the conditions of the environment in which they exist. Because sea shells are composed primarily of calcium, the key factor is whether the conditions are destructive or benign to calciferous materials.
I would surmise that the shells you observed were deposited on the ocean floor when that region of northern California was under the ocean during a previous geologic era. That part of the Earth's crust was eventually uplifted through the forces of plate tectonics, which are responsible for creating the coastal mountain range. If the shells were largely intact and unbroken, its likely that they exist in an primary or original sedimentary layer (not the result of secondary erosion and deposition, the process which is underway now and enhanced by the road cut).
The sedimentary layer in which you found the shells provided a protective environment for the shells, as they were not subjected to substantial physical or chemical weathering processes. That is, until the road cut exposed them. During their protected period they avoided exposure to acidic conditions, for example the infiltration of acid rain, which would have hastened there demise in situ. During this period, they were essentially like any calciferous rock in the Earth's crust. But the road cut exposed them to you, and to the elements. Under these conditions they will eventually degrade and disperse through a combination of physical and chemical weathering. The time length of the process is difficult to predict without a lot of information, but if you consider it on a geologic time scale they'll be gone in an instant.
I had a similar experience as you. I used to live in Santa Barbara on the California coast, and when hiking up a road to the top of the coastal mountains, I found some large shells (like abalone) in the crumbling hillside along the road-cut.
Reply:Fossilized shells can be hundreds of millions of years old.
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