CRITERIA REQUISITE TO A GLACIAL AGE 69 



buried by later glacial deposits, they cannot generally be con- 

 nected demonstrably with the glacial invasions, and are there- 

 fore not here included. The abandoned surface of a recently 

 glaciated region furnishes excellent conditions for the burial and 

 preservation of relics, for it usually presents an uneven surface 

 which becomes dotted with ponds, swales, and ungraded valleys, 

 so that erosion and deposition are the natural order of things. 

 The basins furnish alluring grassy meadows and the ponds offer 

 enticements to drink, while the bordering bogs and springy 

 grounds lie in wait to mire the weak and unwary, and clayey 

 inwash from the higher ground provides a preservative burial. 

 Although the interglacial beds were subject to much disturbance 

 and to removal during the succeeding ice invasion, they were pre- 

 served in a singularly large number of cases, and fossils, often 

 in the most excellent state of preservation, are sometimes found 

 in them. The delicate tissues of mosses are occasionally admir- 

 ably retained. 



The fossils of-these beds have the further merit of definite 

 fixity in time and localization. In this respect there is not the 

 uncertainty of the previous class. The imbedded plants and 

 animals lived subsequent to the deposition of the underlying 

 drift and previous to that of the drift above. 



But even here there is a source of some vagueness, though it 

 is not radical. It is a not uncommon practice of glacial geolo- 

 gists to include among the interglacial formations gravels, sands, 

 and clays, which overlie an earlier bowlder clay, even though it 

 may be quite possible that these were formed in the last stages 

 of the preceding glacial epoch, and are indeed nothing but the 

 glacio-fluvial deposits of that epoch. Of course, this is not 

 done by the discriminating glacialist where the case is deter- 

 minate. But usually the case is not determinate, and then the 

 interglacial epoch is given the benefit of the doubt. Any error 

 in date arising from this source would be a conservative one, 

 since the relics are, at the worst, only referred to a slightly 

 later date than they are entitled to. 



Sources of error connected with interglacial deposits. — The liability 

 to error here is not very serious, if due circumspection is exercised. 



