Understanding the factors controlling fjord morphology is required to assess the impact of glaciations on topography and the dynamics of ice sheets. We investigate the role of lithology on glacial valley form using field observations and numerical landscape evolution models. We measure fjord depths and widths from East Central Greenland (68°-75°N), and find a significant control of lithology on fjord width, with wider fjords in softer rocks (i.e., sediments). This dependency of fjord width to bedrock properties is predicted by a quarrying erosion law, but not by an abrasion one, when considering results from a simple 2D model and a more detailed 3D ice flow model (iSOSIA). Our field and numerical results reveal the strong control of lithology on the width of glacial valleys and highlight the need to consider erosion by quarrying to properly account for this effect.