Grounded ice sheets start floating on the ocean to form floating ice shelves, at a location called the grounding zone. Across the grounding zone, the amount of up-and-down motion caused by the ocean tides gradually decreases. We model the bending of ice caused by the tides and find the stiffness of ice and squishiness of the bed that best match GPS measurements of up-and-down motion taken at Thwaites Glacier. Our model suggests that the first point of lift-off of the ice from the bed doesn't move very much with the tides, but there may be a km-scale area behind this where there is a pocket of water between the ice and the bed. This area could experience fast ice sheet retreat if the ice thins.