Final answer:
The Niobrara formation's cliffs in western Kansas were primarily formed by the deposition of marine sediments over time, particularly from coccolithophore skeletons during the Cretaceous sea.
Step-by-step explanation:
The cliffs of fine chalk of the Niobrara formation in western Kansas were formed by deposition of marine sediments over time. This deposition occurred during the Cretaceous period when the region was covered by a vast sea. Billions of coccolithophore skeletons, which are made of calcium carbonate (chalk), accumulated on the sea floor to form these sedimentary deposits. Subsequent geological processes, including tectonic uplift and erosion, have revealed these deposits as the cliffs we see today. The formation process is similar to that of the renowned white cliffs of Dover, which were also shaped by long-term geological processes rather than by rapid events such as volcanic activity or tectonic activity and continental drift alone.