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Lacustrine deposits are primarily characterized by deposition in lakes, specifically in proglacial lake environments where quiet water conditions prevail. This means that sedimentation occurs in relatively still water, allowing finer particles to settle slowly. In proglacial lakes, which form in front of retreating glaciers, distinct sedimentary features can develop as different materials are carried into the lake by glacial meltwater.

This environment promotes the accumulation of layers of sediment that often contain organic materials and clay, and these deposits can show features such as varves, which are layers that indicate seasonal deposition cycles. The quiet water conditions help create a more uniform and stratified sediment profile, which is crucial for understanding historical climatic changes.

The other options do not align with the typical characteristics of lacustrine environments: arid conditions refer more to desert landscapes rather than lake settings; deep soil layers can occur in many environments, not specifically in lakes; and while valleys may host lakes, they do not directly define the nature of lacustrine deposits. Therefore, the distinct setting of quiet deposition in proglacial lakes clearly identifies the correct answer.