Rocky Cove is an embayment about 1 mile (1.6 km) wide at the base of Step Mountain, between Ursus Cove to the north and Bruin Bay to the south on the western shore of Cook Inlet, about 128 miles (206 km) northeast of Naknek and 76 miles (122 km) west-southwest of Homer, Alaska. The name was published in 1903 by petroleum geologist George C. Martin of the US Geological Survey. Step Mountain forms the headland known as Tignagvik Point. Cook Inlet is a vast basin of Cenozoic rock bounded by older Mesozoic formations along the southern Alaskan coast. A sequence of sedimentary rocks—up to 22,966 feet (7,000 m) thick—fills the basin and is exposed on both sides. These strata serve as key oil and gas reservoirs, with petroleum likely sourced from underlying Jurassic and Triassic formations. On the basin’s western side, Mesozoic and Cenozoic plutonic rocks of the Alaska-Aleutian Range batholith, Early Jurassic volcanic rocks of the Talkeetna magmatic arc, and mainly Jurassic sedimentary rocks eroded from the batholith and the Talkeetna Formation are present. The Bruin Bay Fault runs along the coast, dividing the Alaska Peninsula into a western block dominated by plutonic rock and an eastern block of sedimentary rock. The latter, known as the Naknek Formation, comprises more than 9,843 feet (3,000 m) of sandstone, conglomerate, and siltstone. At Rocky Cove the Naknek Formation appears as dark-gray to black siltstone and calcareous gray sandstone. Deposited in moderately deep water—well below wave base yet above carbonate compensation depth—these sediments formed in a basin with restricted ocean circulation and contain fossils of the pelecypod Buchia and ammonites Amoeboceras, Phylloceras, and Perisphinctes. Sedimentary records reveal repeated Cenozoic glaciations interspersed with estuarine, lacustrine, and alluvial deposition. Late Pleistocene unconsolidated deposits stem from retreating glaciers that once filled the basin with ice. As the glaciers receded and sea levels rose, a saltwater incursion created an ancestral Cook Inlet estuary.

Rocky Cove’s entrance is obstructed by reefs that extend about 2 miles (3.2 km) offshore and are exposed at low tide. The sea cliffs, reefs, and intertidal wave-cut platforms mark an actively retreating coast. Cliff erosion depends on several factors: the rock’s structure—whether igneous, metamorphic, or sedimentary—and its permeability, solubility, hardness, and jointing; exposure to waves, tides, and currents; and subaerial weathering from runoff, freeze-thaw cycles, and chemical processes. Hard rock cliffs, such as basalt or metamorphic rock, retreat less than 0.4 inches (1 cm) annually, while softer sedimentary cliffs like siltstone and sandstone may recede over 3.3 feet (1 m) per year. Retreat is often episodic, with the cliff face collapsing after wave quarrying, abrasion, and weathering undercut its base. Wave quarrying disintegrates rock by generating shock pressures and air compression in small joints and fissures. Wave abrasion scrapes and shatters rock as fragments, driven by wave-orbital motion, repeatedly impact the surface. Weathering results from alternating drying and wetting, salt crystallization in fissures, and chemical processes including dilution and biotic activity. Wave-cut platforms form when destructive waves undercut the cliff between high and low water marks, creating a notch that enlarges until its roof collapses, prompting the cliff to retreat landward. The notch base then becomes the platform floor as collapsed material breaks into smaller particles and is carried away. Adjacent uplands, including Step Mountain, form a sequence of raised marine terraces or ‘steps’ resulting from tectonic coastal uplift and Quaternary sea-level fluctuations. The west side of Cook Inlet is undergoing tectonic uplift and post-glacial rebound, making these terraces prominent even though seismic events—such as the 1964 Alaska earthquake—have caused widespread subsidence.

A wave‐cut platform is a hostile environment for marine life. Constant wave action and sediment transport along sea‐cliff bases and shallow subtidal platforms inhibit the recruitment of benthic organisms, though some species colonize rock platforms and reefs exposed only at the lowest tides. Aerial mapping of Rocky Cove’s intertidal biota reveals a nearly continuous band of brown alga called winged kelp (Alaria marginata). Kelps are large brown algae comprising roughly 30 genera. Algae are photosynthetic organisms traditionally classified as plants, though seaweeds range from large multicellular forms to single-celled species and are grouped into green, red, and brown algae. Evidence now indicates that brown algae evolved independently from nonphotosynthetic ancestors that entered into endosymbiotic relationships with red algae rather than deriving from cyanobacteria; consequently, they are no longer classified as plants. Kelps typically form dense forests in shallow oceans and are thought to have appeared during the Miocene—about 23 to 5 million years ago. Renowned for their rapid growth, species such as giant kelp (Macrocystis) and bull kelp (Nereocystis) can grow up to 1.6 feet (0.5 m) per day and reach lengths of 100–260 feet (30–80 m). Winged kelp grows up to 13 feet (4 m) and bears long, narrow fronds with a distinctive raised midrib and wavy edges. It thrives in wave‐exposed intertidal habitats in the northeastern Pacific—from Alaska to near Point Conception, California—and often forms large patches that support diverse intertidal life. Read more here and here. Explore more of Rocky Cove and Cook Inlet here: