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Geolens

Glossary

417 words used in the labs.

Find a term from the labs, understand the process behind it, and see how it appears in the landscape. Each entry connects an explanation with a real observation and a model you can explore.

Term

A horizon

The mineral soil layer near the surface, where organic matter mixes with sand, silt and clay. Its darker colour often reflects humus, but colour alone does not establish fertility or the depth of the layer.

What to look for

Calciustoll near Ponce, Puerto Rico: a Mollisol with a roughly 30 cm mollic surface over a calcic horizon. Its parent material includes colluvium and weathered limestone.

Related labs

Soil profile →Soil erosion →
A Puerto Rican soil profile has a dark surface above pale carbonate-rich subsoil, with a depth scale.
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Explore further with USGS water glossary · USGS volcano glossary · NPS glacial geology · NASA Earth facts. Photograph acknowledgements are collected on the Image credits page.