TRANSACTIONS OF SECTION C. 683 



producing rarefaction. He concludes by urging geologists to investigate a country 

 which offers such a promisin;,'' field, and does not present to the traveller any 

 difficulties of importance. 



5. On pre-Glacial Valleys in Northamptonshire. By Beeby Thompson, 



F.C.S., F.G.S. 



The paper refers to the pretty general belief that the larger physiographical 

 features of this country were developed before the Glacial period, and that many 

 of the present river valleys are of pre-Glacial age; and remariis that, if so, they 

 ■were more or less completely choked with boulder clay during tlie Glacial 

 period. 



Where a valley got completely filled up it would, perhaps, in many cases be 

 easier for the ordinary drainage to cut out a new valley than to remove the in- 

 filling of an old one, where the initiative for a new valley had been given by 

 superficial streams due to melting ice. So at the close of the Glacial period, 

 although the main drainage of a district must take approximately the same direc- 

 tion that it did before it, the tributary streams would seldom accurately follow 

 their old lines on having, as it were, a i'resli start under somewhat difi'erent cir- 

 cumstances. Compromises would, no doubt, frequently result. 



Bearing these matters in mind, we should be prepared to find — 



1. New valleys without drift, and filled-up old ones near at hand. 

 '2. Valleys witli one side drift and the other the normal rocks of the 

 district. 



3. Valleys still containing much drift, with the streams running over or 



• through it. 



4. Valleys in which only the coarser material of the drift is left in the 



form of river gravel. 



Illustrations of each of the four cases enumerated are given, all from Northamp- 

 tonshire. 



The author suggests that his explanation of some isolated patches of boulder 

 clay near to Northampton may prove to be of more general application than pre- 

 viously suspected. 



I 



6. Notes on some Tarns near Snoivdon. By W. W. Watts, M.A., F.G.S. 



During a recent visit to Snowdon, the writer has taken the opportunity of 

 examining a few of the tarns in its immediate vicinity. These include the two 

 small lakes in Cwm Glas, Glaslyn, a"nd Llyn Llydaw. 



In the hollow of Cwm Glas tbere are two tiny tarns named Ffynnon Freeh 

 and Ffynnon Felen ; both lakes drain over a barrier of rock, but in a rainy season 

 the upper one appears to find a second outlet over the long, low col to the East, 

 so that, in this state, it has the two outlets depicted in the 6-inch map. There 

 can be little doubt that this upper lake is a portion of a bending valley dammed at 

 both ends by scree- and stream-debris, and thus compelled to find an escape over 

 the rocky side. The lower lake is certainly contined in a rock basin, as rock occurs 

 at its actual outlet and at everjj point where any former outlet might have been 

 possible. The lake is, however, so shallow that its occurrence in a basin of rock is 

 perhaps of little consequence. 



The neighbouring hollow of Cwm Dyli, as is well known, contains three lakes, 

 the highest being Glaslyn, the next Llyn Llydaw, and the lowest Llyn Teyrn. 

 Glaslyn is bounded on all sides by live rock except at and near its outlet. This 

 exit is over morauie, which, however, is evidently not very deep, for rock makes 

 its appearance just below, and in such a way as to almost compel belief in a com- 

 plete rock bar. Beside the present course of the effluent stream is a parallel strip 



