CHAP. XT. PHASES OF ALPINE GLACIAL ACTION 321 



4th. A second retreat of the glaciers took place when they 

 gradually shrank nearly into their present limits, accom- 

 panied by another accumulation of stratified gravels, which 

 form in many places a series of terraces above the level of 

 the alluvial plains of the existing rivers. 



In the gorge of the Dranse, near Thonon, M. Morlot dis- 

 covered no less than three of these glacial formations in direct 

 superposition, namely, at the bottom of the section, a mass of 

 compact till or boulder-clay (No. 1) twelve feet thick, including 

 striated boulders of Alpine limestone, and covered by regularly 

 stratified ancient alluvium (I^o. 2) 150 feet thick, made up of 

 rounded pebbles in horizontal beds. This mass is in its turn 

 overlaid by a second formation (No. 3j of unstratified boulder- 

 clay, with erratic blocks and striated pebbles, which consti- 

 tuted the left lateral moraine of the great glacier of the 

 Rhone, when it advanced for the second time to the Lake of 

 Geneva. At a short distance from the above section, terraces 

 (No. 4) composed of stratified alluvium are seen at the heights 

 of 20, 50, 100, and 150 feet above the Lake of Geneva, which, 

 by their position, can be shown to be posterior in date to 

 the upper boulder-clay, and therefore belong to the fourth 

 period, or that of the last retreat of the great glaciers. In 

 the deposits of this fourth period, the remains of the 

 mammoth have been discovered, as at Morges, for example, 

 on the Lake of Geneva. The conical delta of the Tiniere, 

 mentioned at p. 27 as containing at different depths monu- 

 ments of the Roman as w^ell as of the antecedent bronze and 

 stone ages, is the work of alluvial deposition going on when 

 the terrace of 50 feet was in progress. This modern delta 

 is supposed by M. Morlot to have required 10,000 years for 

 its accumulation. At the height of 150 feet above the lake, 

 following lip the course of the same torrent, we came to a 

 more ancient delta, about ten times as large, which is there- 

 fore supposed to be the monument of about ten times as 



