A stratum is a layer of sedimentary rock or soil, or igneous rock that were formed at the Earth’s surface, with internally consistent characteristics that distinguish it from other layers. Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil it is composed of a variable mixture of small particles such as sand, silt and/or clay, but with a much lower percentage of organic matter and humus. Below the subsoil is the substratum, which can be residual bedrock, sediments, or aeolian deposits. Ground Layers with spontaneous natural collapse. Ground strata groundcover cover underground undersoil collapse cave cavity hole earth land cavern pit geology. 4K UHD 3840×2160 30p.