 Complete the following phrase:  Expeditions attempting Everest via the North Col generally arrive up the East Rongbuk Glacier and so do not reach Lhagba La at all. However, when George Mallory and Guy Bullock were trying to reach the North Col, the route from Rongbuk was unknown to them. Instead they approached from the east only to find the glacier did not extend to the North Col. The climbing team eventually had to cross the pass and descend some to the East Rongbuk Glacier before ascending to the North Col. Their discovery allowed the 