85 



It may appear remarkable that so few beds of Tufa have been 

 found in connection with the streams which flow from the 

 calciferous strata of the Cotteswolds, streams which in their 

 higher parts have generally a rapid descent, but this is in 

 consequence of the little resistance presented by the rock beds 

 of our Oolites, not one of them being capable of long maintaining 

 a ledge sufficient to produce a waterfall. On the river Frome 

 and its tributaries more than one hundred mill-falls have been 

 obtained by artificial means ; but it is probable that not one 

 natural fall of their waters existed for any considerable length 

 of time. 



The deposits of Tufa at Chalford and Dursley have not 

 taken place from the waters of the principal streams which flow 

 past those localities, but from the water of springs which from 

 special causes have obtained precipitate falls before joining the 

 main streams. 



At Chalford the rivulet which comes down from the Sapperton 

 valley after passing for five miles over the rock beds of the Inferior 

 Oolite there, cuts through the yielding Oolitic Sands, in which it 

 has a narrow channel. On both sides of the valley, the hills, 

 which rise very steeply, are considerably affected by faults, which 

 probably have much to do with the position of the fine series of 

 springs abeady mentioned, whose waters burst out in such 

 abundance on the south side of the valley at an elevation of 30 

 feet above the level of the stream, and which, in the course of 

 their short passage and rapid descent to join the stream, formerly 

 deposited Tufa in such large quantities. 



From somewhat similar circumstances a fall of water has 

 formerly existed at Dursley. The Cam river, after passing over 

 the Marlstone, there cuts into the more yielding beds of the 

 Lower Lias ; the Broadwell spring, whose waters are derived 

 from rainfall on Stinchcombe Hill, in their passage to the Cam 

 had to pass over the Marlstone, the rock bed of which probably 

 presented a hard ledge sufficient to cause a precijDitate fall, 

 around which the ancient deposit of Tufa was formed. The 

 Broadwell stream has since been diverted so as to form faUs 

 for two mills in its short course before it joins the Cam, but at 



