Moheshkhali and Kutubdia offshore islands together with Matarbari and Sonadia islands constitute a complex and unique geological system on the eastern coast of Bangladesh. These islands are neither deltaic bay mouth bar nor estuarine mouth bar but sub-maridional, somewhat en-echelon and seemed to have a same sub-surface structural trend. Several litho-logs, depth of wave-cut platform and of Tertiary bed rock are considered to reconstruct the palaeo-depositional environments and to unveil the erosional and depositional processes of these islands.

Sedimentary sequences of the northern and north eastern fringe of Moheshkhali coastal plain and northern part of Kutubdia Island exhibit similar characters with those of the Matamohuri estuarine plain which was occupied and protected by the Chakoria Sundarbans in recent past. The present geomorphological context together with thick and repetitive sequences of fine and peaty sediments alternating with sands containing shell fragments and wood debris are indicative to an estuarine environment of a protected depositional system for eastern part of Moheshkhali coastal plain and northern part of Kutubdia Island. On the other hand, sediments of the southern and western fringe of Moheshkhali coastal plain, Sonadia, Matarbari and southern part of Kutubdia islands seem to be an open to semi-protected depositional system. The present shore-faces of these areas are characterized by the presence of numerous long-shore bars along the west coast of Kutubdia and Sonadia and barriers are found to occur from the southwestern edge of Matarbari to the northern tip of Sonadia islands, and along the western margin of the Moheshkhali coastal plain. The presence of remnants of beach rock sequence at the lowest lowwater level at south-west of Baraghop, Kutubdia Island and Oyster reef at mid-west shore of Matarbari Island also establish the open basin depositional system.