FAST BACKWARD: Beaches of Buhangin and Catalunan

The title looks odd but, geologically, the areas of Buhangin and Catalunan, characteristically above sea level, were once part of the ancient beaches of Davao City. The recession of coastal boundaries is influenced by tides and the movement of the sun and the moon.

Weather disturbances also contribute to the rise in rainfall which, in turn, results in heavy downpour that causes erosion. The irregular movement of oceans produces asymmetrical waves that hasten siltation upon hitting the coastline. The impact of strong waves also pushes back water levels in streams and, over time, contributes to the collapse of riverbanks.

Warren D. Smith, a geologist at the Division of Mines in Manila, prepared a narrative of the 1907 of the geologic expedition conducted in Mindanao and Sulu. The report was contained in the 1908 article, “A Geologic Reconnaissance of the Island of Mindanao and the Sulu Archipelago”, published in The Philippine Journal of Science.

Smith, along with geologist H.M. Ickis, explored Cotabato and Davao as part of the multi-group expedition dispersed throughout Mindanao. At Cotabato, he observed a 150-meter-high limestone hill that stood out in the middle of the delta of Rio Grande de Mindanao, known to the Moros as the Pulangi River. Interestingly, the delta composed “all the river plain from the mouth of the river back.” (“Pulangi” in Maguindanao means “large river.’)

The plain which was formerly part of the sea, the geologist observed, could have extended to as far the regions in the gulf Davao. He wrote:

“The most notable feature of the topography of this plain, beside the hill just mentioned, is the old terrace lines which swing along, but not always parallel to, either side of the river. These terraces are undoubtedly of marine origin, for close to Cotabato they are seen to be raised coral-reef shelves with the characteristic steep seaward slope of such formations. The evidence seems sufficient… to suppose that the sea once swept far up this intermontane region which is now filled with sediment…that it one time extended through to the Gulf of Davao.”

Reaching Davao from Fort Pikit on December 20, 1907, Smith’s team, which included an American army lieutenant surnamed Caffery and a unit of cargadores, for the first time got an unobstructed view of the majestic Mount Apo. During this part of the sojourn, he made a very attractive observation that supports his earlier hypothesis.

“A walk back over the plan behind Davao is interesting. About 300 meters behind the town, or about [two] kilometers from the beach, unmistakable signs of old beach lines are found, marked by one distinct terrace at least 15 to 23 meters above the flat on which the town [of Davao] is located. All this territory is made up of alluvial wash from the hills. The bowlders are largely andesitic.” (An alluvium is the “loose, unconsolidated soil or sediments, which has been eroded, reshaped by water in some form, and re-deposited in a non-marine setting.”)

Smith also came out with intriguing details that fit the geologic descriptions of barangays Buhangin (presumably after the sand, which an alluvium) and Catalunan, short of saying the ancient settlers of Davao harvested seashells and basked under the sun on their coastlines.

Arguably, the two regions happen to be situated within two kilometers of the nearest beach. Both areas are about 250-450 feet above sea level, depending on where you are. Terrace formations are observed in land outcrops, especially at Shrine Hill, Juliville Subdivision, Diversion Road, and J.P. Laurel Avenue. Alluvial deposits are in areas within and adjacent to the barangays, including sediments harvested as aggregates in quarries in Davao River.

In many locations, as recorded in numerous chronicles written by explorers and scientisits, seashells, gastropods, corals, and other marine fossils can be found, confirming the assumption these areas were once submerged undersea eons ago. (At Samal, once part of the geologic intercontinental shelf, calcified shells of giant clams have been recovered.)

The Australian Atlas of Mineral Resources, Mines, and Processing Centres explains in simpler terms the presence of sand inland.

“Over millions of years, these igneous and metamorphic rocks were weathered and eroded, and the grains of quartz and other minerals in the rock, including rutile, ilmenite, zircon and monazite, were washed down to the sea by heavy rainfall and fast flowing streams.

“The heavy minerals were then carried back up onto the beach by waves. As the waves washed up and down on the beach, they carried the lighter quartz grains with them back into the sea, leaving the grains of the heavy minerals rutile, ilmenite, zircon and monazite behind on the beach. Wind also helped to concentrate the heavy minerals by blowing away the lighter quartz sand. These processes were repeated many times over millions of years, eventually creating a large deposit of mineral sands on the beach.

“As the sea level rose and fell over geological time, the shoreline moved further inland and then back again. As this happened, the deposits of mineral sand were covered by more sand and built up or eroded and redeposited elsewhere. This is why we sometimes find mineral sand deposits many kilometres inland and maybe as much as 50 metres below the surface.”

An important aspect in the realignment of coastlines is the earth’s movement and the adjustment in its tilt. In today’s phenomenon, the melting of glaciers, which results in the rise of ocean tables, is the result of climate change.