Which unit decreases the movement of water within geological layers?

The unit that decreases the movement of water within geological layers is known as an aquitard. An aquitard is a geological formation that restricts the flow of groundwater to or from adjacent aquifers. It is composed of materials such as clay or silt, which have low permeability, meaning they do not allow water to pass through easily. This property makes aquitards crucial in groundwater systems, as they can confine water within aquifers and create pressure differences that lead to the formation of artesian wells.

Understanding the role of aquitards is essential in hydrogeology, as they influence the behavior of groundwater flow and the management of water resources. In contrast, aquifers are permeable layers that can store and transmit water, while aquicludes are even more impermeable than aquitards, effectively preventing any water flow. Aquifuge, while less common, refers specifically to formations that have no water movement at all. Thus, the aquitard is distinctly characterized by its ability to significantly reduce water movement between geological layers, making it the correct answer to the question.