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Dropstone, California

Pictures of Sedimentary Structures


A large cobble sits embedded in mixed mudstone and conglomerate. Dropstones may have several different explanations. (more below)
A clast out of place
Photo (c) 2010 Andrew Alden, licensed to About.com (fair use policy)
Dropstones are clasts that stand out in a rock because of their unusual size and two other typical features: they depress the bedding beneath them and they cause overlying bedding to drape over them. All three criteria are met in the case of this dropstone, although they are more obvious in the classic case, in which finely laminated, fine-grained deep-sea sediment is disturbed by a cobble.

There are three ready explanations for dropstones, but all three involve something carrying a stone from somewhere else and dropping it. The first is the mechanism called ice rafting, in which icebergs carrying sediment drift into the open ocean and melt, dropping whatever the ice was carrying. The second involves dead trees being washed out to sea that have stones in their roots. The third explanation is that kelp plants are torn off the seafloor and take stones with them clasped in their holdfasts, which rot and release their stones. Because this rock unit is of Cretaceous age, from a period in which glaciers were scarce or absent on Earth, we can rule out ice rafting.

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