Seogwipo Formation is a sedimentary layer formed between 1.8 million years to 550,000 years ago by volcanic activities and environmental changes which resulted in layers of volcanic ejecta and marine sediments. The layer was discovered after thousands of ground water boreholes were studied since the 1960s. One of the major findings of these studies is that the lavas are only 50 to 150 m thick along the coastal regions and are underlain by a 100 m thick volcaniclastic sedimentary formation (a rock formation produced in volcanic eruptions). The volcaniclastic deposits are generally impermeable and act aquicludes throughout Jeju Island, thus controlling the behavior of underground water flows significantly. The volcaniclastic deposits are compared to the exposed portion of Seogwipo Formation. Seogwipo Formation is exposed only in the south central part of Jeju Island along a cliff wall overlain by a 400,000 year old hawaiite lava. The 40-m-thick outcrop (a visible exposure of bedrock on the surface) is composed of mainly basaltic volcaniclastic rocks. The lower half of the exposure is characterized by shallow-marine lithofacies (under 200 m deep from the shore) and consists of alternations of fossiliferous and non-fossiliferous units, whereas the upper half is mostly non-marine ( referring to creatures and organisms that inhabit in land or fresh water environments) and non-fossiliferous. Several transgressions (a geological event during which shoreline moves toward higher ground due to rises in sea level or land sinking) and regressions (a process when areas of submerged seafloor are exposed above the sea level) are recorded in Seogwipo Formation. A variety of fossils are contained in the fossiliferous units of the exposure, including mollusks, brachiopods, foraminifera, ostracodes, nannofossils, sponges, corals, barnacles, echinoids, shark teeth, bryozoans, and whale bonds as well as prolific trace fossils. Because of the fossil diversity, the formation was designated as a natural monument of Korea (no. 195) in 1968. These fossils suggest that the formation accumulated throughout the Early Pleistocene. The outcrop section can be subdivided into 10 fossil-bearing and 4 fossil-free units that are distinguished by either sharp or erosional surfaces in lithology, fossil occurrence, grain size and sedimentary structures. Paleobiolgocial and geological analyses of the sedimentary facies and fossils suggest interplay of a variety of depositional settings including storm-dominated shoreface, with sporadic input of volcaniclastic materials, sandy nearshore to inner shelf, and mud-dominated outershelf. The deposition is interpreted to have occurred under the influence of changing sea levels during alternating glacial and interglacial periods in the past.