Even when buried below ground, archaeological features are sometimes recognizable on the surface thanks to abnormal vegetation or crop patterns for which we use the technical term cropmarks. Growth rate, size, thickness, and color of vegetation are among the clues that can give away the presence of subsurface buildings, fortifications, or backfilled pits.
Especially in the early stages of the growing season and during dry spells, variations in moisture and nutrients present in the ground translate into different patterns of plant growth. Buried ditches, trenches, or pits (including graves) typically support thicker, taller, and faster growing vegetation because they better retain moisture, allow better root growth, and provide more nutrients than buried walls or stone structures. Vegetation in more favorable spots also lasts longer and ripens later in the season. All this can provide clues as to what types of archaeological features may exist beneath the ground.
Deep-rooted and nutrient-demanding, cereals offer the best cropmarks, but legumes and even grass can produce patterns indicative of buried archaeological features.
In other cases, vegetation itself can be considered an artifact, a sign of human-altered environment. Old-growth trees or ornamental bushes—especially invasive or transplanted species—can point to the existence of old roads, house sites, and property boundaries.
Best visible from the air, cropmarks are most effective at revealing subsurface remains when large archaeological features are involved. Evaluation of aerial photographs can inform archaeologists’ decisions, such as where to engage in archaeological excavation.