We can best gain some idea of past time by knowing the agencies at work; and learning how deeply the surface of the land has been denuded, and how much sediment has been deposited.

通过了解工作中的机构， 我们可以最好地了解过去的时间；并了解土地表面被剥蚀的深度， 以及沉积了多少沉积物。