Complex sentence: Often, the sedimentary basin is within rocks that are very different from the sediments that are being deposited, in which the lateral limits of the sedimentary layer will be marked by an abrupt change in rock type.
Split into simpler sentences: Often, the sedimentary basin is within rocks that are very different from the sediments that are being deposited. In those cases, the lateral limits of the sedimentary layer will be marked by an abrupt change in rock type.