The earliest ideas on the creation of potholes are that they were associated with "moulins de glacier" (glacier mills) formed where surface streams on glaciers and ice sheets fall into holes in the ice. Water entering these surficial holes was believed to impact on the bedrock beneath creating a large pothole. The "Moulin Hypothesis", first suggested in 1874, continued to be accepted by many authors until the 1950s. However, commencing in the 1930s, other authors have suggested dissatisfaction with the moulin hypothesis, largely on the grounds that it failed to explain how ice could remain stable long enough for the "giant" potholes to form and why many potholes (like those at Rockwood) were present in large numbers.