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Why is the Precambrian-Cambrian boundary record often incomplete in landward areas?

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Cambrian paleogeography — Cambrian Period Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Cambrian Period. Included in the paleogeographic reconstruction are the locations of the interval's subduction zones.

The incompleteness of the Precambrian-Cambrian boundary record in landward areas is primarily attributed to global fluctuations in sea level.1

During the latest Precambrian, sea levels were relatively low, which resulted in expanded continents and spatially restricted oceans. Because sediment was typically eroded from the continents and deposited in adjacent seas, the most complete sedimentary and fossil records of this transition are generally found toward the outer margins of continental shelves.1

Consequently, as one moves in a landward direction, the time gaps represented by the boundary surface increase. This leads to a significant absence or incompleteness of the transitional record in many areas, particularly those historically favored for classical Cambrian studies. This lack of a continuous record, combined with a historical deficiency in knowledge regarding Precambrian biological communities, contributed to the long-held, though now corrected, misconception that the appearance of Cambrian fossils was an abrupt or sudden event.1