MK. T. M. READE AN ESTIMATE OF POST-GLACIAL TIME. 295 



sidence ? We have really very little evidence to go upon excepting 

 that regional changes of land and sea are extremely slow. If wo 

 put it down at 2 feet per century, the subsidence would he com- 

 pleted in 10,000 years ; let us, however, be generous and say 4 feet 

 per century, which would give 5000 years. 



But then land-movements arc not always going on ; we have seen 

 that the land has been practically stationary for the last 2500 years. 

 If we allow another 2500 years for a pause before the subsidence 

 began, we shall, I think, make a very low estimate. 



The elevation of the estuarine silts (Formby and Leasowe beds) 

 must have preceded the depression, and as they were mainly laid 

 down at approximately the same relative level of land and sea as 

 now obtains, we shall have to allow 5000 years for the elevatory 

 movement, estimated at the same rate as the subsidence *. Working 

 backwards from the present time we arrive at the conclusion that 

 (2500 + 5000 + 2500-1-5000=) 15,000 years is a very moderate 

 estimate for the time which has elapsed since the completion of 

 the laying down of the Pormby and Leasowe estuarine silts. 

 Working still backwards in time, if we allow another 2500 years 

 for the pause during which the estuarine silts underlying the peat- 

 and forest-bed were laid down, a parsimonious estimate I consider, 

 we shall arrive at a total of 1 7,500 years for the time occupied by 

 the Post-glacial changes represented by the Formby and Leasowe 

 and accompanying forest-beds. 



But, as I have indicated, the denudation of the Boulder-clay upon 

 which these deposits lie is very great and represents a much greater 

 lapse of time. Measured in the centre of the valley, at least 100 feet, 

 and probably more, of glacial deposits had been swept out of the 

 Mersey Valley before these Post-glacial beds were laid down. Again 

 we are in a difficulty for a time-modulus to apply to the excavation 

 of the valley, which would be more rapid than the general lowering 

 of average ground, the rate of which may be taken at 1 foot in 4000 

 years t. Let us assume that the valley was swept of its -deposits 

 at the mean rate of 1 foot in 400 years, or 10 times the general 

 mean rate ; the time occupied in the denudation of 100 feet would 

 be 40,000 years. Considering that the Mersey Valley was widened 

 out to nearly its present extent before the estuarine beds were laid 

 down, a mean rate of 1 foot in 400 years is, to my mind, very rapid. 

 As there are no indications'of glacial action during the course of these 

 events, I think that the estimate of 57,500 t years (17,500 + 40,000), 

 say in round figures 60,000 years, for Post-glacial time a reasonable 

 one and, as represented by these changes, well within the mark. 



The calculation includes the time occupied in the elevation of the 

 glacial deposits from beneath the sea, as it is assumed that the 



* There are no known instances affecting so large an area of a rate of 

 elevation or subsidence so great as this. The observations in Sweden showed 

 a mean rise of 3 ft. 6 in. in 134 years. ' Nature,' Dec. 18, 1884, p. 150. 



t *' A Delta in miniature," Quart. Journ. Geol. Soc. May, 1884. " Denu- 

 dation of the two Americas," Presidential Address, Liverpool Geol. Soc. 1884-5. 



I Dr. CroU, from entirely different data, estimates it at 80,000 years. 



