In a survey in 1958, the geologist C. C. Fagg identified 25 active swallow holes between Dorking and Mickleham; the majority were only a few centimetres in diameter and were located in the vertical banks of the river below the water line. Most holes were difficult to observe in times of normal or heavy flow and were susceptible to silting up as new holes were continually being formed. A few much larger swallow holes were also observed separated from the main river by a channel of about a metre. About six of these larger swallow holes were found to the west of the Burford Bridge Hotel, along the course of the A24 Mickleham Bypass during its construction in 1936. Initially the surveyors tried to fill the holes with rubble to prevent the foundations of the new road subsiding. However this proved to be impractical and they were instead covered by concrete domes, up to 18 m in diameter, each fully supported by the surrounding chalk and provided with a manhole and access shaft to allow periodic inspection. In the late 1960s the domes were reopened and inspected and the alluvium in the largest swallow hole was observed to have subsided by under the centre of one of the domes. When the Dorking to Leatherhead railway was constructed in 1859, a fossilised swallow hole was discovered in the cutting at the south end of Box Hill and Westhumble railway station, suggesting that even in its early history, the river had swallow holes.