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When archaeologists excavate a site they might do a sample of small units placed across the site, or they might open several long trenches or a large block of land. Each technique has its advantages and problems.

Unit Excavations
Initial testing of a site might include excavation of a 2 x 2 meter unit, dug in 5 or 10 cm levels, with all soil screened. Units are useful for examining the different layers or strata in the site because the units can examine many levels of the site fairly quickly. Several units might be placed next to each other to follow out on interesting features or get a bigger picture of the site. However, digging a large and deep site is very time-consuming and expensive.

Image of unit excavations.
Unit excavations at the Mill Pond site in Prairie du Chien revealed a series of mussel shell deposits and other artifacts in stratified or layered deposits spanning 1200 years.

Block Excavation
Excavating several units as a continuous area or block gives us a clearer view of the overall plan of the site. Block excavations allow us to see how different parts of the site are related, or to find large structures such as house plans. Large block excavations work best when a site has a single component and does not require deep excavation of multiple layers of occupation.

Image of the main excavation.
This is a portion of a large Oneota village site. The site grid is marked by stakes. Each square is 2 x 2 meters.

Salvage Excavations
With salvage projects, it is often not be possible to excavate the whole site because of time or expense, but a sample of the site is examined. It may be necessary to use heavy equipment to strip the topsoil. This exposes stains, or features, and makes it easier for archaeologists to recover information before a site is destroyed through construction or development.

The Pammel Creek site in La Crosse was excavated in 1989 prior to construction of a flood control system.
Image of a backhoe
Backhoe removing the topsoil over the site area to expose features.
Image showing the excavation.
Excavations in progress.

Image showing the construction area today.
The area today, after construction.

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