Excavating the Feature
Beneath
the parking lot asphalt of the city block at 540 Folsom, arhcaeologists
find a dark, squarish smudge of organic soil. As dark soil can be formed
by the decomposition of organic material, it is suspected that this
is the top layer of a privy deposit. The feature
is called "Privy 507". 
Hollow features such as privies can be filled by several processes: by their primary use, i.e. layers of excrement, by silts and sands from flooding or rain run-off, or by dumping objects such as household rubbish. These processes create layers, also known as contexts, that can be differentiated by their color, texture or density of artifacts. The sequence of layers within a feature is called its stratigraphy and it can reveal important information about how a features such as Privy 507 were created.
Although the top of Privy 507 was partially destroyed by the construction
of 
a
brick footing, its stratigraphy is undisturbed. The archaeologists set
to work with trowels and buckets. As excavation of each of the contexts
within Privy 507 is begun, the archaeologist assigns it a unique identifying
number. The material from the contexts is piled into buckets, each tagged
to keep the soil and artifacts of different contexts separate. The soil
is screened through mesh with 1/4 inch-holes to recover the artifacts.
Contexts thought to contain small seeds or fishbone are screened with
1/8 inch mesh. Artifacts found in each context are sent in labeled paper
bags back to the lab for washing, reconstruction, and cataloging.
Not every archaeological feature is worth the time and expense of fully
excavating and processing. Some features have been too 
disturbed
by animals, later buildings, or bottle collectors. Others contain very
small numbers of artifacts, or cannot be associated by historical records
with a known historic household or business.
How does the archaeologist decide in the field, under the pressure of time which archaeological features are valuable enough to excavate?
The mnemonic AIMS-R
or “Association, Integrity, Materials, Stratigraphy and Relative Rarity”
summarizes the criteria by which a feature is assessed. Can it be reliably
associated with a historic household or business within an identified
period of time? Does it have good stratigraphic definition? Is 
it
relatively undisturbed and its stratigraphy intact? Does it contain
a good number and variety of artifacts, or does it represent a type
of household, ethnic group, industrial process or time period for which
very few other archaeological remains are known?
Privy 507 is a pretty easy call. Apart from the disturbance of the brick footing to its upper layers, the privy’s stratigraphy is intact. Judging from its artifacts, it dates to the early 1870s, a time period that is poorly represented amongst already-excavated archaeological deposits. Historical research suggested that it was created by the Peel family who occupied that parcel from 1856 to 1879.
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