Maiv — Tufa Deposits in Flintshire. 253 



IV. — On the OccuKKEisrcE of Extensive Deposits of Tufa in 

 Tlintshiiie. 



By George Maw, F.S.A., F.L.S., F.G.S. 



|F the various geological evidences inferriBg a vast duration of 

 the existing contour and conditions of surface since the latest 

 marine submergence, there is, perhaps, none more striking than the 

 accumulation of great masses of Tufa charged with recent organic 

 remains. 



Whether the extreme slowness with which limestone is capable of 

 undergoing watery dissolution, or the smallness of the volume of 

 the water, which has generally been the medium of its transfer to a 

 tufacious condition is considered, any great mass of Tufa must of 

 necessity imply an immense lapse of time for its formation. 



The Mountain Limestone district of North Wales contains several 

 examples of Tufa deposits, and one, to which my attention has been 

 drawn by my friend the Eev. D. Williams, of Nannerch, seems, 

 from its position and great extent, to be worth a special notice in the 

 Greological Magazine. 



At the. south side of the great Mountain Limestone range of 

 Flintshire, connecting the plain of Chester, near Mold, with the vale 

 of Clwjrd, opposite Denbigh, runs a long, narrow, tortuous valley, 

 from which branch numerous ravines, intersecting the limestone 

 range, forming its northern boiuidary. 



One of these ravines, immediately to the east of Caerwys, and a 

 second, about a mile further to the east, are occupied for a consider- 

 able distance with extensive deposits of soft Tufa, commencing 

 against the Mountain Limestone, and expanding downwards through 

 a range of altitude of about 150 feet in broad delta-shaped masses 

 towards the main valley, where the Tufa flow appears to have been 

 arrested on the north bank of the river Wheeler. 



The more westerly of the two ravines is occupied from north to 

 south for a distance of three-quarters of a mile, and the total area 

 occupied by both masses cannot be less than 200 acres. I beheve 

 that deposits of Tufa at all approaching these in extent are un- 

 known in any other part of the kingdom. 



At the embouchure of the eastern ravine, close to the Smell turn- 

 pike-gate, a cutting in the new Mold and Denbigh railway has 

 exposed the deposit for a distance of more than a furlong, and in 

 one place to a depth of 12 feet. At the point in digging the founda- 

 tions for a bridge a further depth of six feet was penetrated, and a 

 bed of peat, one or two feet thick, containing fragments of semi- 

 decayed wood, found to intervene between it and the subjacent drift- 

 gravel. The thickness of the Tufa would therefore be 18 feet. 

 Higher up the lateral valleys it ascends in terrace-like ranges, and 

 appears to be much thicker as the streams have cut channels into 

 it 15 or 20 feet deep without exposing the fundamental drift. 



For the most part the Tufa is of a soft marly texture, occasionally 

 containing harder masses, which appear to have accumulated round 

 plant remains ; the harder parts are full of tubular perforations, 



