BRITISH POSTGIACIAL S- RECENT DEPOSITS. 389 



bedding. From the inclination or dip of the deposits, and the 

 lie of the gravel-stones, it is evident that they have been laid 

 down by a current of water flowing persistently down the 

 valley, in other words they are true fluviatile accumulations. 

 They prove, therefore, that the sea eventually retreated, and so 

 allowed streams and rivers to plough their way down through 

 the thick sheets of clay, sand, and gravel, which had been 

 spread out by the floods of late glacial times, and had gathered 

 over the floor of the old estuary. They further tell us that the 

 land must have stood at that time at a higher level than it does 

 now, for we find them here and there passing below the present 

 beds of the rivers. How deep they go I cannot say, but various 

 phenomena lead me to believe that their actual thickness cannot 

 be great. Probably the ancient postglacial bed of the Tay 

 does not lie more than a few fathoms below the bottom of the 

 present stream, and thus the old coast-line at the period I refer 

 to may not have stretched much farther out to sea than it does 

 now. But however that may have been, this we do know, 

 that the ancient Earn and Tay ploughed a deep and broad 

 course through the late glacial deposits, which when the sea 

 had retreated must have extended at first as a broad and 

 approximately level plain over all the lower reaches of the two 

 valleys. Through this plain the rivers cut their way to a 

 depth of more than 100 feet, and gradually removed all the 

 material over a course which can hardly be less than two miles 

 in breadth below Bridge of Earn, and is considerably more 

 than that in the Carse of Gowrie. Thus we are not only assured 

 that the land then stood at a somewhat higher level than now ; 

 but we are compelled to conclude also that a very long time 

 must have elapsed between the disappearance of the old estuary 

 and the accumulation of the buried forest. Eor incoherent 

 although the late glacial deposits are, and incapable of resisting 

 the powerful erosion of running-water, yet a prolonged period 

 of time was necessarily required for all the denudation they 

 experienced before the trees of the buried forest began to grow. 

 As the ancient river-deposits have not as yet yielded any 



