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Sam A Johnstone


I am a Research Geologist interesested in understanding how processes of erosion and faulting interact to shape Earth’s surface. I study these phenomena through geologic maps, laboratory techniques that measure the history of rocks being exhumed toward the surface, and with measurements and models of Earth's surface topography.

I came to the USGS as a Mendenhall Postdoctoral Fellow in 2016 to study landscape evolution in the Southern Rocky Mountains and to help complete a facility for measuring the cooling histories of minerals as part of the Mineral Resources Program. In 2018 I transitioned to a job as a Research Geologist funded by the National Cooperative Geologic Mapping program, where I work now. In this capacity I conduct regional-scale geologic mapping and synthesis of Quaternary sedimentary deposits with the goal of ultimately establishing a seamlesss national-scale geologic map database that can aid in the characterization of geologic hazards and natural resources.  This work is currently focused in two regions, in Arkansas and in a large transect spanning the Intermountain West, for which I also serve as the lead of the surficial geologic working group. 

In addition to these roles I conduct research into the mechanisms of landscape evolution in an effort to better interpret the geologic record of natural hazards and past environmental change, focusing on two broad classifications of problems.  First, I seek to develop numerical and statistical models to refine geologic inferences made from geochronologic data and to quantify uncertainty in those interpretations.  Second, I try to understand the connection between earth surface processes, landscape form, and recent geologic deposits by developing numerical models that enable simulation of these phenomena.  In these efforts I am interested in developing approaches for characterizing landscape evolution through paired analysis of topographic and geologic map data.

Prior to starting at the USGS I obtained my PhD from Stanford University in 2016, where I studied how sediment transport and erosion processes shaped hillslopes and river networks, and how some micro-climatic gradients enduced by aspect differences influenced these processes. In 2011 I obtained my MSc from UC Santa Cruz, where I had also obtained my BS a few years earlier. It was at UC Santa Cruz that I first got excited about geologic mapping and where I developed an interest in using low-temperature thermochronology to study histories of faulting.