Complex sentence: Below this is a layer of yellowish-red, gravelly, silty clay loam, which extends to approximately below ground and lies over a layer equal portions of gravel and silty clay loam.
Split into simpler sentences: Below this is a layer of yellowish-red, gravelly, silty clay loam, which extends to approximately below ground. Under this is a layer that contains equal portions of gravel and silty clay loam.