200 DR. A. JOWETT ON THE [June I914, 



extent to which it has been altered by the action of the weather 

 since its deposition. Tough blue clay, being impervious, weathers 

 to yellow clay for only a little distance below the surface, whereas 

 a sandy clay may be yellow down to 3 or 4 feet or more. On 

 the moorland, the weathered portion of the Drift is usually white 

 or greyish white, owing to the removal of the iron -oxides by the 

 peaty acids in the percolating water. 



The texture of the Drift, by determining the extent to which 

 rainwater can percolate through it, has had an important influence 

 upon the apparent distribution of limestone boulders. In nearly 

 every good section of tough impervious Boulder Clay, smoothed 

 and striated Carboniferous Limestone boulders abound, accompanied 

 by angular fragments of Carboniferous chert. In porous Drift 

 and in the weathered portions of Boulder Clay near the surface, 

 limestone-boulders are generally absent, but chert is almost always 

 present. Thus, because of its insolubility and its wide dissemi- 

 nation, chert has proved of great value to the investigator in 

 working out the distribution of the Drift, when the limestone, which 

 was also present originally, has been removed by solution. 



(b) Constituent Materials. 



Most of the materials of which the Drift is composed are such as 

 may be derived either from the rocks below or from those which 

 are, broadly speaking, immediately on the north or west of the 

 deposit. A variable, but generally very small, proportion of the 

 materials does not occur as solid rock within the drainage-area in 

 question. Moreover, in the lowlands the Drift is of a more 

 generalized type, being less dependent upon the nature of the solid 

 rock upon which it rests, and having its matrix more thoroughly 

 ground up than in the uplands, where it includes a greater proportion 

 of local material and approximates more to ordinary rock-waste, the 

 nearer one approaches the upper limit of its distribution. 



The matrix of the Boulder Clay, consisting as it does almost 

 entirely of locally-derived material, varies in colour from chocolate- 

 brown and red, where the New Red Sandstone rocks occur, to blue- 

 black where Coal-Measure shales crop out. Numerous sections may 

 be seen where soft rocks pass almost imperceptibly upwards into 

 Drift — the only changes noticeable being the gradual destruction 

 of their stratification, and the presence of occasional boulders of 

 some other rocks which have been ploughed into them. 



But for the exceptional occurrence of erratics of large size, the 

 biggest boulders in the Drift consist of local rocks, the far-travelled 

 stones being usually smaller, more rounded, and more frequently 

 striated. Well- striated fragments of black shale and even coal are, 

 however, by no means of rare occurrence. 



Three well-marked types of Drift may be distinguished 

 by the boulders which they contain. 



The most abundant boulders derived from local rocks are coarse 

 and fine sandstone, shale, coal, and ironstone from the Millstone 



