296 MR. T. M. READE AN ESTIMATE OP POST-GLACIAL TIME. 



denudation went on pari passu with the elevation. It may be urged 

 that the mean lowering of the ground in a Boulder-clay area would 

 proceed much more rapidly than in average ground composed of a 

 variety of rocks. I am not prepared to admit this. Boulder-clay is 

 a very tenacious substance, and, if only attacked from the top surface, 

 takes long to denude. 



The valleys would be excavated at a greater rate than in rock, 

 provided there were a sufficient gradient, as the clay is more readily 

 undermined. This I have allowed for in the increased rate of valley- 

 excavation. No ordinary rock would be worn down so quickly as 

 1 foot in 400 years. Again the rate of vertical excavation by a 

 river would not be uniform ; it would proceed with greater rapi^ty 

 at first and decrease as it reached the base-level of erosion, then it 

 would cease entirely. 



All this time the valley would be widening and would continue 

 to widen, though the vertical excavation ceased. Other things being 

 alike, the widest valley must be the oldest. The Mersey Yalley is 

 distinguished for its width in the upper estuary, and, in this, tidal 

 action has helped. When we look at the Eiver Ribble below 

 Eibchester, we cannot help being impressed with the idea that 

 there has been little change since the Eoman times. I found in 

 1882 Roman tiles 3 feet below the surface-soil, on the river-cliffs, 

 resting on the Boulder- clay. There are Boulder-clay cliffs on either 

 bank, so that the lateral movement of the river at this part since 

 Boman times cannot have been great. 



It is quite apparent that there has been in the area under con- 

 sideration considerable general denudation of the Boulder-elay. 

 Bocky knolls once covered have been laid bare. Here and there 

 we find patches of Boulder-clay as indications of its former pre- 

 sence. The surface of the clay below the soil is full of ramified 

 channels and holes filled with the sand left from the destruction of 

 the clay above *. Unfortunately there are no zones in the clay by 

 which we might be enabled to say how much of it has been removed 

 in particular areas. We are thus driven to the valleys for an 

 answer. They are in many cases remarkably wide, with Boulder- 

 clay at the bottom and on the plateau above, while the flanks often 

 display the rock surface. This is the case at many points in the 

 Mersey Yalley and in its tributaries. 



When we consider that at the rate of 1 foot in 4000 years the 

 mean general lowering would only amount to 10 feet in 40,000 

 years, we may well consider our estimate based on valley-denudation 

 a moderate one. 



When first investigating the Post-glacial deposits of south-west 

 Lancashire and Cheshire, I was much impressed with the great lapse 

 of time they indicate ; and a long acquaintance with this class of 

 geological investigation has not lessened the impression. 



As I have before stated, in none of the Post-glacial beds have any 

 remains of extinct animals been found. If at a future time any 



* See " Subsoil Denudation of Boulder-elay," Geol. Mag. 1882, p. 265. 



