Forming and testing historical hypotheses:: _{G.K. Gilbert and the geology of the Henry Mountain}

This material is excerpted from a teaching module on the Visionlearning website, to view this material in context, please visit Research Methods: Description.

In the United States, description was used as a scientific research method extensively during the exploration of the west. In the 1860s, the U.S. government launched a series of four surveys into the American west: the King, Wheeler, Hayden, and Powell surveys lasted from 1860 until 1879. Each of these survey teams included a geologist whose job was to observe and describe the landscape. The geological exploration of the Henry Mountains in southern Utah provides insight into the research method of description and one of its most respected practitioners, G.K. Gilbert.

On his two trips down the Colorado River in the 1870’s, John Wesley Powell had noted the Henry Mountains with interest. They lay west of the river as it wound through Glen Canyon, rising abruptly from the surrounding mesas. Their dark peaks contrasted sharply with the light-colored, layered rocks that surrounded the river on all sides (see the photo in Figure 2). He did not have time to explore the mountains himself, and thus gave the task to Grove Karl Gilbert, a young geologist who had worked with him previously. Gilbert spent a week in the mountains in 1875, and returned for a month in 1876, emerging with several notebooks filled with carefully recorded notes, sketches, and hypotheses. In August of 1875, he wrote the following observations: “…The rock which rises towards Hillers from the south is the B Cliff. It is lost in the ‘debris’ without increasing the dip (7°) with which it approaches. But beyond are red and white sands – [deeper] rocks tilted almost to the vertical and interspersed with dikes. Moreover these sandstone hogbacks seem to trend in a curve around the mountain as far as they extend.” He made a sketch, shown in Figure 3 below, in which he indicated where he had taken measurements and summarized his descriptions.

Gilbert collected samples of each of the rock types labeled in Figure 3 and described them. The layered rocks (labeled b, c, d, s, and p in Figure 3) were all sedimentary rocks that he had seen before in his travels, and he described them by color, grain size, and the fossils they contained. The core of Mt. Hillers was different, however: an igneous rock formed from cooling magma. He described this rock as “a pale gray paste with large white crystals of feldspar and crystals large and small of hornblende.” Based on this composition he determined it was trachyte, labeled t in Figure 3. Gilbert sketched the same mountain from several different angles and collected measurements on the orientation of rock layers all the way around it, while also collecting basic survey data like elevation.

Just a few notebook pages and one day later, Gilbert takes the first leap from observation and data collection to hypothesis. He sketches a cross-section that is no longer a faithful depiction of the landscape, but is a hypothesis for the formation of Mt. Hillers (Figure 4). This initial model takes all of his observations thus far into account: the gentle slopes of the sedimentary rock units leading up to the peak, the steeply dipping rocks that continue all the way around the mountain, and the igneous rock at the center. He devotes little text to explaining his idea at this point, writing only about a “reservoir” of magma forming below the surface and deforming the rock layers above it. He knew he had more data to collect in order to test this initial hypothesis; the next day, he wrote in his notes, “I don’t understand the NE side of Hillers.” He could not yet explain what was going on there through his initial hypothesis.

Gilbert had only a few days left in the Henry Mountains that summer as he continued on his exploratory journey. The next year (1876) he returned, however, to the task of collecting more data to revise his initial, simple hypothesis. After a month of collecting survey data and rock unit orientations, Gilbert and his team were stuck in camp because of rain and snow. Gilbert took the opportunity to summarize his findings and display the development of his thinking over the course of his explorations. He re-drew the sketch from the previous year and wrote an explanation for his new model: “The simplest type of Henry Mt. structure is a lenticular mass of trap [an early word for igneous rock] above which the strata were arched… The form of the trap mass is never fully shown but it can be described in a general way in several cases… Injections are combined and grouped variously. A trap mass with sheets above and below may be regarded as a system of exudations from one chimney.” He includes a more realistic representation of the process as it applies to the Henry Mountains specifically (Figure 5). Gilbert then poses a question about these intrusions: Why didn’t the magma reach the surface?

Gilbert published his “Report on the Geology of the Henry Mountains” in 1877 (Gilbert, 1877). In the publication, he named this newly discovered type of intrusion a laccolite (later changed to laccolith). He formalized his field sketches, measurements, and research questions into a coherent theory about the formation of the Henry Mountains and noted that many other mountains in the region were likely formed through the same process. He proposed two different answers for his question about the underlying causes of the intrusion relating to relative densities and the “penetrability” of the sedimentary rocks, but his question has still not been resolved today. Gilbert’s report is considered a key milestone in geology, not only for the new type of intrusion he proposed, but for the highly systematic way in which he used the descriptive method of science; as a result, Gilbert went on to become a highly valued and respected leader of the new U.S. Geological Survey and two-time president of the Geological Society of America.

Gilbert’s work also laid the foundation for further research on the Henry Mountains. More detailed measurements of the folding and faulting in the rock layers gave strong support to Gilbert’s laccolith hypothesis (Jackson & Pollard, 1988). More recently, researchers have experimented with physical analogue models to try to understand why the magma did not make it to the surface by building sedimentary layers out of sand and injecting the layers from below with silica gel to simulate magma (Roman-Berdiel, Gapais, & Brun, 1995).