The Breeding Birds of Central Lower California

Abstract

By wagon road and burro trail it is an even one hundred miles from Santa Rosalia, on the Gulf of California, to the tide line of the Pacific at San Ignacio Lagoon. The intervening country is essentially a desert. The summit, which is two thousand feet high in the passes and nearly three times that altitude in the mountains, lies within ten or twelve miles of the Gulf. The two watersheds are thus of unequal length. They are also of quite distinct configuration. On the eastern side the descent to sea-level is abrupt and precipitous, checked by two rather extensive valleys. The long western slope, on the other hand, is broken by valleys, cations, and pretentious hills. It is marked with the weird formations which are characteristic of arid North America and which are here exaggerated. Angles and profiles of silhouetted hills and table-tops are unusually harsh and forbidding. There is not even the softening effect of grandeur. Over the major portion of the entire region lava has flowed and mesa, valley and mountains are covered with dull brown rocks. This lava sheet, though of varying thickness, normally does not exceed three feet in depth. In overlaying the ancient sandstone it parallels the slopes of the hills and the sides of the cations, while on the mesas, and sometimes in the valleys too, it is as level as they and often stretches away as far as the eye can see. The lava covering, in the process of cooling, has broken into fragments that rarely exceed a cubic yard in size. The edges are sharp and the lines of cleavage easily traceable. Indeed, so slightly has the deposit weathered, it seems as though but yesterday it solidified and cracked. The lava surface is not present everywhere, but it does cover four-fifths of the area we examined. Where it is absent it presumably either has been removed bty water or covered with ashes or sand. An exception is the western margin of the cross section where the ocean and lagoon bottoms have changed position in recent geological times. These are either smooth and salt-crusted or else they have been worked into sand dunes. There are several systems of dry river beds which have an important influence on the biology of this region. Even though the country be arid, beyond anything known in the United States there still is enough rainfall to provide some moisture. This water, as well as a part of that from the cloud-bursts that come once in a decade, finds its way to the sea by means of a subterranean flow. The conspicuously marked stream courses and the accompanying level valley floods are outstanding features of the landscape. The alluvial deposits are usually river sand, but they also include extensive beds of cobble stones or of relatively fertile silt. Occasionally the subterranean flow encounters bed rock formations which force the water to the surface. On the Gulf side, three miles west of Santa Agueda, there is an oasis which supplies water for Santa Rosalia and its suburbs as well as for numerous truck gardens. On the Pacific watershed, where the drainage lines are longer, we found natural surface water at San Joaquin, the Alamo, and in Jose Maria Cation. Outstandingly the most extensive of the oases is San Ignacio, where a town of a thousand people is supported. The settlement contains truck gardens and orchards in addition to a jungle of date and fan palms.