A very interesting case of a lake basin, formed by cutting off an arm of the sea without any movement of the earth's crust, is the Salton Sink of southern California. This basin, many miles long and wide, lies below sea level, its lowest point being 287 feet below tide. The Gulf of California formerly reached much farther north and into California where it covered the site of the Salton Sink. Gradually the Colorado River, always loaded with sediment, built a broad delta deposit right across the gulf, the northern end of which thus became cut off, leaving a big salt lake. But the river flowed into the gulf, while in the dry climate the evaporation was great enough to gradually dry away the salt lake. This was the condition of things until 1904, when much of the river at a time of flood got out of control and, following the general course of a great irrigating canal, it flowed for several years into the lowest part of the Salton Sink, partly filling it to form a lake 45 miles long, 17 miles wide, and 83 feet deep. Since 1907 the lake has been notably decreasing in size, and it may entirely disappear. Other ways by which lakes, mostly relatively small ones, may develop are by landslides blocking valley drainages; by streams cutting across winding curves leaving so-called "oxbow lakes" which are common, for example, along the lower Mississippi River; by wave and wind action along shores of lakes or sea; by filling so-called "sink holes" which result from dissolving or falling in of roofs of caves; and by beavers through whose industry dams are built across valleys or streams. Some of the most common ways by which lakes may be destroyed are the following: by being filled with sediment carried in by streams, or by vegetation, or by both; by cutting down outlets; by evaporation due to a change in climate; by removal of the ice dam in certain types of glacial lakes; and by movements or warping of the earth's crust. CHAPTER XII HOW THE EARTH MAY HAVE ORIGINATED The problem of the origin of the earth is essentially astronomical rather than geological, because geological history is considered to have begun when common earth processes, such as erosion, deposition, and transportation of sediments, etc., were brought into play. It is quite certain, however, that the earth in its pregeologic state gradually merged into its geological condition. For this reason the geologist is interested in the more important doctrines or hypotheses which have been put forth to account for the origin of the earth. In fact, one of the few hypotheses which must be taken seriously is largely the work of a geologist. The most acceptable hypothesis not only best satisfies the facts regarding the earth's astronomical relationships, but also best harmonizes with our knowledge of the oldest known rocks and their history.