Final answer:
The defining soil horizon of a Gley Soil is the B horizon, where fine material accumulates and forms a dense layer in the soil.
Step-by-step explanation:
In soil classification, a defining soil horizon of a Gley Soil is the B horizon. The B horizon is an accumulation of fine material that has moved downward, resulting in a dense layer in the soil. In some Gley Soils, the B horizon may also contain nodules or a layer of calcium carbonate.