[pct-l] Addendum: Dave Toms on snowbridges

[rellinwood at worldnet.att.net (Robert Ellinwood)]

Dr Bob:
Doggone it, Dave, I missed one topic I meant to follow through on. You 
do mountaineering...   Help me with snowbridge evaluation and crossing
techniques.  I have always been extremely cautious (read: chicken) 
about trusting snowbridges.  In the Sierra, I once killed several hours 
climbing upstream to desperately find a spot to cross, rather than cross 
a snowbridge that even had footprints on it, for gosh sake!   Ah, but 
had the sun totally changed the tensile strength of the arch since 
those 110 lb hikers crossed?  What are the things/conditions to look 
for? Maybe it's always a gamble, but what tells you the odds are in your 
favor in crossing?  

Dave:
They're a pig to evaluate.  As a rule of thumb, if I can't punch an ice 
axe shaft through it (hold vertically and plunge very firmly once), it 
will hold me. You're right to worry about the sun; the difference in 
strength just an hour's sun can make is immense, as witnessed every 
time you posthole on snow that only 30 mins before you were gliding 
effortlessly over :-)  If I can punch a skipole with basket on through 
the crust, it is unlikely to hold me.

Even if I can get the axe through, it will still sometimes hold. Of course, 
if we're on a mountain we're roped and its a bit safer. We try and get 
off crevasse fields etc. before midday.

Amusingly, Michele, despite being way lighter than me, sinks in more - 
wrong surface-area to weight ratio on her feet I think.  She went 
into one waist-deep Sierran lake, and when mountaineering, usually 
postholes worse than me.