THE IMPORTANCE OF FIELD WORK, WITH AN EXAMPLE FROM G.K. GILBERT’S EXPERIENCE IN THE BONNEVILLE BASIN
In 1879 Gilbert spent a few days studying exposures of Lake Bonneville sediments at the Old River Bed, in west-central Utah, and described what he referred to as the “wedge locality,” where he noted a layer of gravel that thickened in a wedge between two fine-grained stratigraphic units. The rounded gravel clasts in the wedge are composed of volcanic rocks, limestone, and quartzite, derived from outcrops and alluvial fans nearby, and the gravel is encrusted and cemented by tufa. At the wedge locality there are good exposures of the two major stratigraphic units defined by Gilbert, the yellow clay, which is flat-lying and underlies the gravel, and the white marl, which is draped over the gravel body. In his field notes Gilbert interpreted the gravel as lacustrine (he called it a “bar”), but in his monograph on Lake Bonneville (published more than a decade later) he interpreted the gravel as alluvium. Aerial photographs, which of course weren’t available to Gilbert, show that the gravel forms a lacustrine spit, so Gilbert’s field interpretation was correct, and his office interpretation was off the mark.
The point of this example is that the field observations and interpretations of an excellent geologist were more accurate than the interpretation he formulated long after he had seen the field relationships. Gilbert’s published interpretation was part of a misconception about Lake Bonneville history that was carried forward in the scientific literature for at least a century. The lesson: don’t underestimate the importance of good field observations and interpretations, and resist the temptation to replace field interpretations with other thoughts that conflict with basic data from the outcrop. References: Gilbert, 1890, USGS Monograph 1; Hunt, 1982, BYU Geol. Stud. 29.