Teamwork has been identified as an essential employability skill and learning outcome in the geosciences, especially during fieldwork. Although specific teamwork skills have been identified in prior research, few studies to date have addressed how students develop or use these skills during their educational preparation in science, technology, engineering and mathematics (STEM) disciplines, including the geosciences. In this paper, we use a descriptive, qualitative embedded, single-case study to explore how geoscience students develop teamwork skills during a hydrogeology field course in the absence of any explicit instruction about teamwork, using a theoretical lens of input-process-output taxonomy of teamwork skills. We collected data using the Geoscience Teamwork Observation (GTO) protocol and verified findings against focus group discussions held after each week of team observations. We identified that students do use a wide range of teamwork skills in the absence of instruction on teamwork, and these skills changed across different teams and under different contexts during fieldwork. Student teams most frequently used skills of communication, leadership, peer-mentoring and teaching, and coordination. Skills related to goal identification, information synthesis and organizational management were utilized least often. We recommend instructional strategies that explicitly treat teamwork skills as learning outcomes, prior teaching of teamwork as part of fieldwork, and using observations as a strategy for teaching and assessing teamwork during fieldwork. We also provide a shared approach for evaluating teamwork skills to enhance workforce preparation and draw attention to key issues relating to creating effective teamwork outcomes during fieldwork.