70 THE FRESH-WATER LOCHS OF SCOTLAND 
The microbarograph at Ardtrostan shows a somewhat gradual 
drop of 2 mm., followed by a sharp rise of 4 mm. between 15^' 44™ 
and 16^ S™. 
It does not appear that either the passage of the main depression, 
or the minor fluctuation attending it, could have caused the sudden 
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Fig. 24. 
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initial rise shown on the limnogram at 16^' 9 '6™. Both of these 
causes would indeed have worked, if at all, in the opposite direction. 
We are therefore driven to the probable conclusion that the 
uninodal seiche was caused by the flood. A glance at a map of the 
neighbourhood shows that the area — Glen Beich, Glen Ogle, Glen 
Droma, Glen Ample, and Glen Voirlich — which drains into the 
western half of I^och Earn much exceeds that — Glen Tarken, Allt an 
