A dip is a gentle depression in an otherwise ﬂat landscape in which rain or snowmelt gathers seasonally. Neither a pond nor a marsh, which are longer lived, a dip is still sufﬁciently different from the surrounding terrain to create its own small-scale ecology born of subtle changes in moisture, nutrients, and sedimentation. Dip, dip slope, and angle of dip all refer to the degree to which a surface, as in a layer of rock embedded in a geologic formation, deviates from the horizontal. Dip has another sense as well: A compass needle aligns with the lines of the Earth’s magnetic ﬁeld. When the compass is brought close to where these force lines converge, at the north and south magnetic poles, the pivoting needle is also pulled down from the horizontal. This magnetic dip further complicates directional navigation in the far north and south.

D. J. Waldie