Jeju Island comprises extensive lava flows and hundreds of monogenetic volcanic cones with rare and thin sedimentary deposits. However, a number of boreholes reveal that the lavas are only 50 to 120 m thick along the coastal regions and are underlain by a 100 m-thick volcaniclastic sedimentary formation, which is correlative with the Seoguipo Formation. Detailed examination of the subsurface Seoguipo Formation, as recovered from the Sagye borehole in the southwestern part of the island, reveals that the formation consists of primary hydrovolcanic facies (massive or inclined stratified lapilli tuff) emplaced by Surtseyan fallouts and debris flows in subaerial or subaqueous settings and reworked hydrovolcanic facies (stratified, massive, or normally graded pebble conglomerate and sandstone) and nonvolcanic facies (homogenous or laminated mudstone) that were deposited in subaerial to submarine (nearshore to offshore) settings. The predominance of volcaniclastic deposits in the formation demonstrates that the early stage of Jeju volcanism was characterized by repetitive hydrovolcanic activity probably because of an abundance of external water for hydroexplosion. The Seoguipo Formation is envisaged to consist of multiple, superposed hydromagmatic volcanoes with intervening, marine or nonmarine sedimentary sequences. This finding warrants redefinition of the Seoguipo Formation as a sedimentary succession, above the U Formation, composed of fossil-barren and and fossiliferous, subaerial and subaqueous, volcaniclastic and nonvolcanic sedimentary deposits beneath the plateauforming lavas.