370 TAYLOR POST-GLACIAL CHANGES IN ITALIAN AND SWISS LAKES 



delta cones of mountain torrents. The torrents descend from high moun- 

 tains ; their fall is rapid, and in time of freshet they carry large quantities 

 of coarse detritus. 



Lake Maggiore attains a depth of nearly 2,800 feet, Como over 1,900 

 feet, and Garda upward of 1,000 feet. The subaqueous walls of their 

 basins are in some places nearly vertical, in a few places overhanging, 

 and their slope is generally as much as 30 degrees, and is often much 

 more. Such steep slopes afford a poor foundation for delta building. 



The deltas are relatively steep conical forms, resembling somewhat the 

 alluvial cones of our arid western regions. They are, in fact, compound 

 forms, partly alluvial cone and partly delta. Every torrent of any size 

 has a conspicuous old delta deposit of this kind at its former mouth, and 

 in most of these old deltas the present stream has cut a deep trench in 

 order to reach the lake at its present lower level. In a few cases the old 

 delta has been so warn away by erosion that it was difficult or impossible 

 to find any certain evidence of a former higher lake level. 



The Structure of Torrent Deltas 



As is well known, deltas are built of sediments arranged in foreset and 

 topset beds. By continual additions of foreset beds at its outer margin 

 or front, a delta grows in horizontal extension ; by the addition of suc- 

 cessive layers to its top it gradually aggrades or builds up its surface to 

 higher levels. The principal growth is accomplished by the foreset de- 

 posits, especially where the offshore slope is steep. As the area filled 

 in by these beds grows larger it forms a platform on which the topset 

 beds are laid down. When the topset beds are composed of very coarse 

 materials, the shape they give to the subaerial part of the deposit may 

 resemble a detrital cone. The torrent deposits of the Italian lakes are 

 mostly of this character. 



In the original process of building the old deltas, when they were 

 aggrading, the streams did not enter the lakes through deep trenches 

 in older delta deposits as they do now, but in shallow distributaries 

 choked with coarse detritus and frequently shifting their places. The 

 coarser sediments were mostly dropped in the headward parts of the 

 distributaries and added to the building up of the detrital cones. The 

 greater part of the finer sediments were carried down to the lake to be 

 added to the foreset beds. The coarseness of the detritis and the steep- 

 ness of the surface slope both decreased toward the lake, so that at the 

 shore the material was mostly pebbly gravel and sand and the slope was 

 more nearly horizontal. A small quantity of sand and fine gravel was 

 deposited at the mouths of the distributaries on the shallow water mar- 

 gin of the deltas, and shore currents and wavelets distributed and re- 

 arranged them to some extent along the delta fronts. 



