156 



CONTORTED STRATA IN DRIFT. 



[Ch. XIL 



all, exactly as if a glacier or iceberg had passed over them and scored 

 them in a manner similar to that which the solid rocks below the 

 glacial drift have so often undergone. It is possible, as Mr. Geikie 

 conjectures, that this second striation of the boulders may be refer- 

 able to the second era of drift or floating ice.* 



Contorted Strata in Drift — In Scotland the till is oftei] covered 

 with stratified gravel, sand, and clay, the beds of which are some- 

 times horizontal and sometimes contorted for a thickness of several, 

 feet. Such contortions are not uncommon in Forfarshire, where T 

 observed them, among other places, in a vertical cutting made in 

 1840 near the left bank of the South Esk, east of the Bridge of Cor- 

 tachie. The convolutions of the beds of fine and coarse sand, gravel, 

 and loam, extend through a thickness of no less than 25 feet perpen- 

 dicular, or from b to c, fig. 137, the horizontal stratification being 

 resumed very abruptly at a short distance, as beyond /, g. The over- 

 lying coarse gravel and sand a, is in some places horizontal, in others 



Fig. 137. 



Section of contorted drift overlying till, seen on left bank of South Esk, near Cortachie, in 

 1840. Height of section seen, from a to d, about 50 feet. 



a. Superficial sand, with some beds of coarse gravel with cross bedding in parts — 4 feet. 

 &, c. Contorted beds 25 feet in vertical height, by the side of which, in the same continuous 

 section, are seen horizontal beds of stratified drift, some of them with coarse gravel and 

 large boulders, 

 c, d. TJnstratified red till, with large boulders of granite, gneiss, quartzite, &c, 20 feet thick, 



the red loam being derived from triturated old red sandstone. 

 d, d'. Similar till continued, thickness unknown. 



e. Inclined strata of old red sandstone, not laid open in this place. 



it exhibits cross bedding, and does not partake of the disturbances 

 which the strata b, c have undergone. The underlying till is exposed 

 for a depth of about 20 feet ; and we may infer from sections in the 

 neighborhood that it is considerably thicker, and that it rests on the 

 edges of highly inclined strata of old red sandstone, as represented in 

 the section. 



* Geikie, ibid. p. 68. 



