Northern Guam geospatial information server

Cliffs in northern Guam

Cliffs are significant vertical or near vertical exposures of bedrock. In Guam, they are composed of limestone. Much of the perimeter of northern Guam is delineated by sheer cliffline that abuts the ocean or a narrow coastal terrace. In general, cliffs are products of erosion (which can leave behind walls of relatively resistant rock) or form as escarpments (where rock bodies undergo vertical displacement along geologic faults). In Guam, however, the vast majority of cliffs started as coral reefs. They originally formed underwater and became walls of limestone when they were tectonically elevated above sea level. The height of cliffs around the northern Guam plateau is variable. This is because the plateau has not been elevated evenly and is tilted north-to-south. The cliffs’ extent above the sea level reaches 160-180 m in the north at Ritidian and Pati points, 70 m at the popular tourist lookout at Amantes Point, and 30 m farther south in Tamuning.

Guam has had a complex geologic history. The original landmass was created by submarine volcanism, which started over 40 million years ago and ceased about 15 million years ago. Most of southern Guam is the result of that volcanic activity. In contrast, northern Guam formed by reef growth. A few million years ago, all of the north was a shallow lagoon surrounded by a barrier reef. Sometime in the past 2 million years, tectonic forces within the earth’s crust caused major uplift that pushed the lagoon up to 150 m above the sea level. That lagoon became the northern Guam plateau, and its barrier reef became the surrounding cliffline. The uplift was rapid but episodic, which is reflected in the step-like pattern of cliffs and terraces in some areas. The terraces represent former sea-level stillstands.

Cliffs provide niche habitats for plants and animals able to take advantage of such structures. Some plants prefer to grow on inclined or vertical limestone bedrock and are adapted to associated soil and water deficiencies, wind exposure, and sea spray. One of Guam’s most beautiful flowers, gausali (Bikkia mariannensis), grows from cracks in rock faces. Other hardy plants able to survive on cliffs are halophytes and xerophytes characteristic of coastal scrub.

Text above is slightly modified from "Environments of Guam" by Danko Taborosi, David R. Burdick, Claudine M. Camacho, Frank Camacho, published by BessPress, 2013.


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