Taganak is the largest island, with an estimated land area of 116 ha. It also has the highest relief with its maximum elevation of 148 m ASL (USCGS, 1940). The substrate of Taganak is basalt, with coralline sand at the southern end. Soil cover of the hilly part is red clay derived from the volcanic bed rock. The coastal plain is covered by coralline sand.

Groundwater in Taganak is available both in the volcanic substrate as well as in the coralline sand flatlands. Water in the volcanic rocks is available in the weathered portion. Water stored in the fractures of the basalt apparently recharges water along the coastal area.

Sandy coastline rims the flatlands at the southern end of Taganak, while shingles beaches abound in small coves (Pallang). Coralline rubbles make up the shingles deposit of these coves. Around the northern end, the coastline is generally rocky. A short stretch of mangrove coastline used to be found in the western side, particularly in Sitio Bakkao. The beach area is generally narrow and in most parts, the beach is bounded by steep slopes of the volcanic hills. It is only in the southern part of the island that the beach and backshore widen due to the presence of a wide coastal plain.

Erosion scars are evident at the southern end of Sitio Limao-limao and the border area of Brgy. Dambilah, while shore erosion at the western side of the southern end of Bakkao has affected a number of houses along the shore.

Bare slopes and landslide scarps indicate high rates of soil erosion in this part of Taganak. A layer of reddish silt is observed along the coast of Sitio Bakkao.

This is the second largest island in the Turtle Islands Group with an estimated land area of 76 ha. The shape of the island is elongated along the northeast-southwest direction. It has a low relief with topography varying from flat to low hills at the northeastern end. Maximum elevation in the island is estimated by USCGS (1940) at 59 m ASL.

The substrate of Boan is sandstone. The continued extrusion of mud by diapiric activity built up the hill of the island. However, it was reported by residents of the island that outflow of mud has already ceased for a number of years now.

The flat part of the island is made up of accumulation of coralline sand, coralline rubbles and some sandstone boulders. This alluvial deposit makes up the aquifer of the shallow groundwater in the island which is essentially freshwater (lense) floating on sea water. The groundwater is directly recharged by rainfall. Large amounts of groundwater are withdrawn from the area by commercial fishing vessels that regularly use the island as anchorage.

The coastline of Boan is dominated by rocky and shingles beach. The shingles deposit is made up of coralline rubbles. A small patch of mangrove is located at the northeastern side of the island. The sandy part of the coastline is now occupied by settlements. Shoreline retreat is evident in the island. Fences of round logs have been set up in some parts of the sandy beach to protect houses from shore erosion. Several berms made up of coralline rubble have been mapped at the northeastern part of Boan.

This is the third largest island in the Turtle Islands Group. It has a land area of about 51 hectares. Its terrain and geology is akin to Boan. Its hilly portion was built up by the extrusion of mud by the mud volcanoes, while the flatland is an accumulation of coralline sand and rubble. As estimated by the USCGS (1940), its highest elevation is 58 m ASL. Vent of the mud volcano was reported to have shifted a number of times in the past. As witnessed during the April 1998 and the October 1996 fieldwork, mud volcano’s activity is limited to quite pulsating extrusion of pure grey mud accompanied by gas bubbling. Dark-colored streaks which appear to be oil film, sometimes accompany the extruded mud. Gas was sampled during the April 1998 fieldwork for analysis.

The northern coastline of Bakkungan is dominated by rocky shoreline. Sandy beach is limited to the southern part of the island. Settlements in the island proliferate in the area fronted by sand beach. Like the other islands, shore erosion seems to be very active in this island. Beach attrition has already affected a number of houses as well the barangay hall of Bakkungan. Land cover pattern of this island is very similar to Boan where agriculture is done in woody areas. It also has a sizeable coconut plantation. Fresh water is present in Bakkungan in the form of shallow groundwater.

Baguan has an estimated land area of 29 hectares with a total coastline of 1,717 meters. The island is volcanic in origin, later enlarged by the deposition of the coastal plain at the southern part. The volcanic part of the island is elevated with a maximum elevation of about 40 m ASL. The hill is moderately steep.

The coastline at the foot of the hills is rocky, with large volcanic boulders, while in the southern part, the coastal plain is surrounded by wide sandy beaches. Erosion and accretion is observed to be active in the island. A long term change in the island’s outline has been observed. Fresh water is present in the island in the form of shallow groundwater.

Lihiman is approximately the same size as Baguan with its land area of approximately 29 hectares. Topography and geology of Lihiman is similar to Boan and Bakkungan. The island is elongated in shape along the northeast-southwest direction. The circular portion at the northeast end was built up by the extrusive activity of the mud volcano. But unlike in Boan and Bakkungan, mud extrusion in Lihiman is relatively more violent creating a crater of more than 20 meters at its hilly portion. Extrusion activity is said to be accompanied by tremors. Extruded material in Lihiman contains a mixture of mud and angular boulders. Dried mud at the top of the agoho trees rimming the crater shows the high pressure at which mud is vented. Due to the huge volume of material discharged, a drainage opening has been carved on the northern hill slope leading to the sea.

The northeastern end of Lihiman is lined by a rocky shoreline, while the beach at the southeastern part is a shingles beach made up of gravel to boulder size rocks. Erosion is observed to be active in this part of the island as evidenced by toppled trees and erosion scarp. Sandy beach dominates the southern and western shoreline.

Groundwater is present in the sandy flatland. Numerous shallow wells have been dug as source of water for the agricultural plots being cultivated by the residents.

Siltation at the northern shoreline is evident. Grey silt deposit, same color as the mud coming from the mud volcano, is observed in the foreshore. It is reported by some local residents that submarine mud extrusion has been observed offshore of Lihiman. This activity is indicated by highly turbid water.