OCR Text |
Show GEOI/ OGY. 13 the lake from its old shores the streams which formed the deltas have begun their destruction by cutting them in two in their progress toward the shrunken body of water. The Bonneville is the most conspicuous of all the shore lines, not because of the relative duration of time during which it was formed but because being the topmost of the series, it emphasizes the contrast between the sharply carved subaerial erosion features of the main land and the broad horizontal lines due to the influence of the lake. Study of the levels of bars at this stage shows that the record is complex and that the water surface alternately rose and fell a few feet during the formation of the shore phenomena that mark the general Bonneville level. Below the Bonneville there are a number of plainly marked shore lines which represent stages in the level of the lake when it was practically constant for relatively long periods. Of these shore lines the Provo is the most remarkable, for it records the longest occupancy of one approximate horizon of any of the stages of the lake. Its embankments are the most massive and its wave- cut terraces are the broadest, notwithstanding the fact that the lake at the Provo stage was considerably smaller than when the surface of the water was 375 feet higher, its area having shrunk from 19,500 to approximately 13,000 square miles. The Provo shore line is characterized particularly by its deltas, which were formed at the mouths of all the larger streams that entered the lake. The fall from the Bonneville to the Provo level was apparently without interruption and comparatively rapid. But below the Provo stage there are remnants of shore lines and terraces at a number of horizons that record temporary halts of greater or less extent in the gradual shrinkage of the lake. The most conspicuous of these lower shore lines, at an elevation of approximately 750 feet below the Bonneville level, has been named the Stans-bury shore line, from its prominent development on Stansbury Island, but the others have not been correlated. As many as ten distinct shore lines can be traced on the west side of Jordan Narrows. In connection with the different shore lines it is of interest to note that Gilbert has found evidences at a few localities of oscillations of the lake level between the Provo and Bonneville horizons, which appear to record halts in the rise of the lake as it approached its maximum. This is unusual, for most of the observed shore phenomena were formed during the retreat of the lake. Local deposits of calcareous tufa occur associated with the various shore lines, but are most abundant at the Provo horizon. The tufa appears to have been deposited by precipitation from the lake waters due to aeration of the waves, especially during storms, and consequent loss of carbon dioxide by which the carbonate of lime was held in solution. The tufa occurs as a cement to gravel and as a more concentrated deposit, from a few inches to a few feet in thickness, coating exposed surfaces. Below the Provo horizon, lake beds consisting of subhorizontal or gently lakeward- sloping sediments are associated with shore deposits until, as the valley bottom is approached, shore markings become indistinct and the lake beds prevail. The deposits of yellow clay and white marl previously mentioned as being widely distributed in the Bonneville basin apparently are not typically developed in the bay of the old lake, which occupied the area under consideration. A number of deep wells have been sunk into the valley deposits and their records indicate the general composition of the sediments ( PI. V). The beds are at least 2,000 feet thick, and consist of gravel, sand, and clay, which constitute the reservoirs in which ground water is stored. CLIMATE. Weather observations have been systematically recorded at Salt Lake City for thirty-one years, and at near- by stations, including Provo, Thistle, Heber, and Park City, for eight to fourteen years. The most important meteorologic data, compiled from reports of the United States Weather Bureau, are summarized in the following tables, which give details of precipitation, temperature, wind velocity, humidity, and evaporation, on which the supply of underground water directly depends. |