W. H. Hudleston — Recent Wells in Dorset. 247 



across the basin through Holme instead of through "Wool, shows 

 a width of nearly nine miles, as against the 4 A- miles through Wool 

 or Bovington. 



"When we come to study the geological position of the Bovington 

 Borehole, the measurements of the Borehole section, in conjunction 

 with the surface plotting based upon the ascertained contours, enable 

 us to obtain a fairly accurate conception of the prevailing conditions. 

 The true axis of the tectonic syncline (see Fig. 4) can only be fixed 

 approximately, but in this case it probably almost coincides Avith the 

 axis of the Prome valley, which here runs very nearly in the trough 

 of the syncline. On the meridian of "Worgret near "Warehara, on the 

 other hand, the bed of the Frome lies a long way south of the synclinal 

 axis. Generally speaking, the southern limb of the Dorset S3'ncline is 

 shorter, and therefore steeper than the northern limb. In this case 

 a northerly dip of 6° is assigned to the south limb above Burton Ci'oss, 

 on the strength of dips observed in the Chalk near Wool. According 

 to statements previously made in this paper, there is reason to believe 

 that the northern limb in the neighbourhood of the borehole has a dip 

 from 1'5° to 2° southwards, taking a sort of average, although there 

 may be subsidiary folds within the general syncline. This is in 

 conformity with the general rule that the clip of the northern limb is 

 at a lower angle than that of the southern one. 



From the Bovington Borehole to the margin of the Chalk escarp- 

 ment near Bulbarrowis a distance of about twelve miles in a northerly 

 direction, and if we deduct three miles for areas covered by Tertiaries 

 along this line, there remains nine miles of Chalk outcrop on which 

 the rain may fall directly to feed its underground waters. It is well 

 known that a large percentage of the water which falls on this 

 absorbent formation sinks into it instead of flowing over the surface as 

 is the case with clays. Moreover, the rainfall on the North Dorset 

 downs is much heavier than in the Frome valley, and still more so in 

 comparison with Weymouth. Hence there is an abundant supply in 

 the region lying to the north of the borehole, and assuming the 

 preponderance of a southerly dip, which may be taken for granted, 

 this water in its underground passage is bound to find its way south 

 until its progress is arrested by the pressure of water from the opposite 

 limb of the syncline. Meanwhile it is kept down by Tertiary beds, 

 etc., having a thickness of 280 feet. Taking the average elevation of 

 the base of the Chalk between the Dorsetshire Gap and Bulbarrow at 

 650 feet ahove O.D., this may be accepted as the elevation at the 

 outcrop in the escarpment of the North Dorset Downs. On the 

 supposition that the Chalk is 900 feet thick beneath Bovington, this 

 would bring the base of the Chalk to 1,010 feet beloiv O.D. at the 

 borehole. Adding these two sums together, we obtain 1,660 feet as 

 the difference in height of the base of the Chalk between the one 

 point and the other. This sum of 1,660 feet vertical has to be 

 distributed over a horizontal distance of twelve miles, and this shows 

 an incline of 1 in 38 = about 1'5°. Thus from the results of an 

 independent calculation, we obtain precisely the same amount of dip 

 for the northern limb of the syncline as had previously been deduced 

 in another way. (See p. 245.) 



