The islands of both groups are rugged and mountainous, and sea caves are common. Their geologic structure is related to that of the Coast Ranges, the Santa Barbara group constituting a continuation of the Transverse Ranges and the Santa Catalina group a continuation of the Peninsular Ranges. Ocean basins and troughs between the islands reach depths of some 6,000 feet (1,800 metres). The islands are noted for their distinctive plant and animal life (several hundred varieties), including many indigenous species. Of note are the giant coreopsis, or sunflower tree, and the endemic island gray fox. Chamise chaparral, grasses, wildflowers, and coastal sage are the characteristic vegetation. Only Santa Cruz and Santa Catalina are forested, covered with pine trees. The islands are the breeding grounds for the California sea lion, several species of seals, and a great variety of seabirds.

The Channel Islands were once home to two Native American peoples: the Chumash in the Santa Barbara group and the now-extinct Gabrielino in the Santa Catalina group. The islands were visited in 1542 by the Portuguese navigator Juan Rodríguez Cabrillo (Cabrillo, Juan Rodríguez), who is reputedly buried on one of them. The larger islands were later used for sheep and cattle ranching, but those activities have ended. Santa Catalina Island is a noted resort, and San Clemente is used as a training ground by the U.S. Navy. Seeping oil from a ruptured underwater well in the Santa Barbara Channel caused widespread ecological damage to several of the northern islands in 1969.

Santa Barbara Island and the Anacapa group were designated a national monument in 1938; these and the three other islands became the national park in 1980, covering an area of 390 square miles (1,010 square km). About half of its surface area is water, since the park's boundary extends l nautical mile (1.9 km) from the shore of each island. The park's headquarters is in Ventura on the mainland.

San Miguel, the westernmost of the park's islands, is administered by the U.S. Navy. It comprises a windswept tableland with a rocky coast, and its climate is often rainy and foggy. Santa Rosa Island is leased by its former owners for game hunting; the remains of Pleistocene pygmy mammoths have been excavated there. Santa Cruz Island has two rugged ranges (rising to Mount Diablo at 2,450 feet 【747 metres】 in the north), a central valley, and year-round streams and springs. The western nine-tenths of the island is owned by the Nature Conservancy, a private environmental organization. Sites of Chumash Indian habitation dot the island. Two of Anacapa's islets consist of plateaus bordered by sea cliffs hundreds of feet in height; the cliffs are a major nesting site of the endangered California brown pelican. Santa Barbara Island, the southernmost island of the park, has steep cliffs rising to a marine terrace with twin peaks.

archipelago in the English Channel, west of the Cotentin peninsula of France, at the entrance to the Gulf of Saint-Malo, 80 miles (130 km) south of the English coast. The islands are dependencies of the British crown (and not strictly part of the United Kingdom), having been so attached since the Norman Conquest of 1066, when they formed part of the duchy of Normandy. They comprise four main islands, Jersey, Guernsey, Alderney, and Sark, with lesser islets and a labyrinth of rocks and reefs. They are administered according to local laws and customs, being grouped into two distinct bailiwicks of Guernsey and Jersey, with differing constitutions. Alderney, Sark, Herm, Jethou, Lihou, and Brecqhou are Guernsey's dependencies, and the Ecrehous rocks and Les Minquiers are Jersey's. The last two were the source of long-standing dispute between England and France until 1953, when the International Court of Justice confirmed British sovereignty. In the late 20th century, the dispute revived, as sovereignty of these islands determines allocation of rights to economic development (specifically, petroleum) of the continental shelf.

Fine scenery, flowering vegetation, and a mild maritime climate have made the Channel Islands popular resort areas. The islands, the only British territory to endure German occupation during World War II, are famous for their breeds of cattle and for the export of fruit, flowers, tomatoes, and early potatoes. They enjoy tax sovereignty, and their exports are protected by British tariff barriers. English and French are commonly spoken (though use of the latter is declining), and a Norman-French patois survives. St. Helier (Saint Helier), on Jersey, and St. Peter Port (Saint Peter Port), on Guernsey, are the islands' main population centres. Area 75 square miles (194 square km). Pop. (2001) 149,878.