Sentence:  The earliest ideas on the creation of <WSD>potholes</WSD> 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 <WSD>pothole</WSD>. 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" <WSD>potholes</WSD> to form and why many <WSD>potholes</WSD> (like those at Rockwood) were present in large numbers. 
Meaning of the word marked with <WSD> tags:  A pit formed in the bed of a turbulent stream.