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.
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.
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.
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.