150 



C. L. MOEGAN- OF THE S.W. EXTENSION 



Before reaching Hill Farm the depression ceases or widens out 

 somewhat indefinitely. This is what we should expect. For since 

 the line of fault cuts across the strata obliquely, thus cutting lower 

 and lower beds of the Mountain Limestone, and since the fault shows 

 signs of dying out in this S.W . direction, a point must be reached 

 when, instead of Fpper Limestone Shales being faulted down 

 against limestone, beds of Mountain Limestone are faulted down 

 against lower beds of the same rock. And since it is only by the 

 bringing in of the softer beds that the line of depression is produced, 

 when these beds are no longer brought down to the present surface- 

 level by the fault, the line of depression ceases. 



I thought it worth while to calculate the distance at which the 

 fault would cease to bring down Upper Limestone Shales to the 

 present surface-level, on the supposition that it shows no tendency 

 to die out in this direction. Supposing that the 600 feet of Upper 

 Limestone Shales are just faulted down in the Leigh Woods, on the 

 Somersetshire bank of the Avon, the breadth of surface covered by 

 these beds, at a dip of 27^°, would be about 1300 feet. The strike 

 of the strata is 65° east of north, and the direction of the fault is 

 70° east of north. The fault therefore cuts the strata at an angle of 

 5°. At such an angle the 1300 feet of Upper Limestone Shales would 

 gradually thin out, wedge fashion, as we pass westwards along the 

 line of fault, — the point of the wedge, where the Upper Limestone 

 Shales cease to come to surface, being about 4300 yards from the 

 Avon. Eut Hill Farm, where the depression before alluded to 

 ceases, is, according to Mr. Sanders' map, not more than 2200 yards 

 from the Avon. The difference I believe to be evidence of the 

 dying-out of the fault in this direction. 



Beyond Hill Farm there is no means of tracing the fault. A 

 quarry a little beyond the farm, at e (see map), shows no sign of shaly 

 or sandy beds. But it is worthy of note that if we produce the line 

 that is marked by the Upper-Limestone-Shales depression, in a west- 

 south-west direction, until it cuts the junction of Mountain Lime- 

 stone and Lower Limestone Shales, we find at this point a great con- 

 fusion of dips. Within a couple of hundred yards or less we find 42° 

 E.I^.E., 2^"" S.S.E., 22° S., 10° W., 10° E.N.E. StiU this represents 

 but poorly the 1150 feet of throw on the Gloucestershire bank of the 

 Avon. If the fault have not practically died out as such, we should 

 expect, here in the neighbourhood of the junction of the softer 

 Lower Limestone Shales and the harder Mountain Limestone, some 

 more marked evidence of its existence. The only further evidence 

 of this kind which I have found, and I do not wish to lay any great 

 stress upon it, is this : that at a point a little to the north of the con- 

 fused dips, in the midst of the line of depression that marks the 

 Lower Limestone Shales, there is an island of Mountain Limestone, 

 at /on the map. Owing to its existence Mr. Sanders, in his map, 

 has carried his Mountain Limestone boundary-line outside this, so 

 as to include it in the Mountain Limestone. But I would suggest, 

 as possible, that this island of Mountain Limestone in the depression 

 is the remnant of a wed^e of this rock faulted into the Lower Lime- 



