This paper is the first chapter of a comprehensive report on the ground-water features in the southern part of the coastal plain in Los Angeles and Orange Counties, Calif., with special reference to the effectiveness of the so-called coastal barrier--the Newport-Inglewood structural zone--in restraining landwar,-1 movement of saline water. The coastal plain in Los Angeles and Orange Counties, which covers some 775 square miles, sustains a large urban and rural population, diverse industries, and intensive agricultural developments. The aggregate ground-water withdrawal in 1945 was about 400,000 acre-feet a year, an average of about 360 million gallons a day. The dominant land-form elements are a central lowland plain with tongues extending to the coast, bordering highlands and foothills, and a succession of low hills and mesas aligned northwestward along the coastal edge of the central low- land plain. These low hills and mesas are the land-surface expression of geologic structure in the Newport-Inglewood zone. The highland areas that border the inland edge of the coastal plain are of moderate altitude and relief; most of the ridge crests range from 1,400 to 2,500 feet in altitude, but Santiago Peak in the Santa Ana Mountains attains a height of 5,680 feet above sea level. From these highlands the land surface descends across foothills and aggraded alluvial aprons to the central lowland, Downey Plain, here defined as the surface formed by alluvial aggradation during the post-Pleistocene time of rising base level. The Newport-Inglewood belt of hills and plains (mesas) has a maximum relief of some 500 feet but is widely underlain at a depth of about 30 feet by a surface of marine plantation. As initially formed in late Pleistocene time that surface was largely a featureless plain. Thus the present land-surface forms within the Newport-Inglewood belt measure the earth deformation that has occurred there since late Pleistocene time and so are pertinent with respect to structural features that influence the watertightness of the so-called coastal barrier. The hills and mesas of the Newport-Inglewood belt are cut by six gaps through which tongues of the central lowland extend to the coast. The gaps are trenched in the deformed late Pleistocene surface and are floored with alluvium that is highly permeable in its lower part. The Long Beach-Santa Ana area, with which this report is concerned, encompasses the central and eastern segments of the coastal plain, and includes five of the gaps in succession from northwest to south- east: Dominguez, Alamitos, Sunset, Bolsa, and Santa Ana Gaps. In the Long Beach-Santa Ana area a thick sequence of Quaternary and Tertiary sedimentary rocks has been deposited on a basement of metamorphic and crystalline rocks of pre-Tertiary age. In the broad syncline underlying tl e central part of Downey Plain these sediments probably exceed 20,000 feet in thickness. This report pertains chiefly to the geology and water-bearing character of the rocks that underlie the coastal zone of the Long Beach-Santa Ana area. This area extends some 27 miles from Dominguez Hill on the northwest to Newport Beach on the southeast, has an average width of about 6 miles, includes some 180 square miles, and borders the Pacific Ocean. Of the Quaternary deposits the youngest are of Recent age and comprise silt, sand, gravel, and clay, chiefly of fluvial origin; they are the latest contributions to the alluvial cones of the Los Angeles, San Gabriel, and Santa Ana Rivers; their thickness is as much as 175 feet. The upper division of the Recent deposits, largely fine sand and silt of low permeability, commonly furnishes water only to a few wells of small yield; the lower division is coarse sand and gravel deposited chiefly in two tongues extending respectively, from Whittier Narrows through Dominguez Gap and from Santa Ana Canyon through Santa Ana Gap.