Screenwashing (Down and Dirty)

A follow up to yesterday’s post.

As mentioned yesterday, large bones and fragments are easy (relatively) to find in the field. Collections include turtle shell, ungulate jaws, rhinoceros and even crocodillian skulls. Hidden within the same layers we find these bones, however, are even more fossils. These fossils, which include teeth and bone fragments of rodents, bats, and other small vertebrates, are often the size of the sediment grains in which we dig, if not smaller.

In order to find these micro-specimens, we must separate bone from sediment.

Bags of sediment, collected from sites in which we have found other, larger fossils.

Sediment bags are labeled with locality ID and collection date.

We separate trash from treasure in a process known as wet sieving, or screenwashing. Sediments are soaked thoroughly and run through a series of wooden boxes with mesh screen bottoms of increasingly smaller sizes. Running the samples through these sieves allows dirt and smaller grains of rock to fall through the screens, leaving behind larger fragments.

The interns prepare a bag of sediment for washing.


Soaked sediments are deposited into the first of three screens. Large rocks will be left behind, while small fossils and clay sized particles are washed down into the next layer.

Not every location in which fossils are found is necessarily a good location for screenwashing. Sites comprised of well consolidated rock can hardly be used – instead a site is needed that consists of soft sedimentary rocks such as mudstone, claystone, and siltstone. These can easily be broken down and washed.


PCP PIRE 9 (5)  PCP PIRE 9 (3)

PCP PIRE 9 (14)PCP PIRE 9 (9)

Following the washing, sediments are lain down to dry.

Jorge Moreno, PIRE intern supervisor, holds a collection of crocodile teeth found in a preliminary search of washed sediments.

Once dry, sediments are bagged and labeled so they can be sent for further sorting and inspection under a microscope.

Sediments from the sacks, washed and labeled, awaiting inspection under microscope.


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About Michelle

I am a STEM (Science Technology Engineering and Mathematics) undergraduate student adamant in my desire for two things - to conduct research, and to participate in and promote science outreach for the general public. I am currently working towards a Bachelor of Science in Geology, with a minor in Geography. Apart from my job as a full time student, I work as a fossil preparator at the John D. Cooper Center (Orange County's Paleontological and Archeological Repository) and as a STEM Ambassador at the Univeristy Center for Careers in Teaching.

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