THE REUSS. 
217 
they appear, sometimes on one side of the valley, 
sometimes on the other. These conditions indicate 
successive phases in the history of the valley — 
periods of comparative quiescence between others of 
more rapid excavation. The two most conspicuous 
are the present river course and the 1500 metre 
terrace. The other two represent former limits of 
ice action, the 2000 metres the highest level to 
which the glacier attained; the 800 metre terrace 
the level of the valley when the ice finally re- 
treated. 
Rtitimeyer proceeds to consider the causes which 
gave rise to the phases of rapid action and relative 
repose, in which respect he regards the Goschenen- 
thal as particularly instructive, and he concludes that 
the periods of repose represent those of great extension 
of the glaciers. This explanation may apply to what 
I would call the two glacial terraces, but not I think 
to the others. The 1500 metre terrace is probably 
the level of the valley at a former time when the 
river had acquired its regimen, and ran for a long 
period at this level. Then came a subsequent eleva- 
tion; excavation recommenced, beginning at the lower 
end, and is still in progress; hence the difference 
between the slope of the terraces and that of the 
present valley. 
