My partner and I are both comfortable multi pitch trad climbing at VS, and have climbed long alpine routes at AD+, however most of our climbing is either in the Lake District or the Alps. Apart from this incident we had an excellent day out climbing Observatory Ridge and descending the CMD arête. Around half way up Observatory Ridge, while moving together after pitching the first four pitches, two very large blocks gave way beneath my feet. The blocks narrowly missed my second climber and fell down into Zero Gully with a lot of loud crashing and rock dust, alarming climbers on the North East Buttress. Thankfully there were no climbers in Zero Gully or at the base of the route at the time. I managed to shout out a warning to those below as the rocks fell. The climbers on North East Buttress called over to make sure we were not injured. The blocks fell as I was stepping up and I already had a hand on the very good holds above, so I did not fall. We had two pieces of gear on the 20m of rope between us so hopefully had I fallen I would have been held.