Yes it flows downhill, but as long as ice is accumulated in the accumulation zone, which is in the upper region of the glacier, the downflowing part is filled u

Yes it flows downhill, but as long as ice is accumulated in the accumulation zone, which is in the upper region of the glacier, the downflowing part is filled up by new ice from above. By the way the article doesn't say that the veins were uncovered by the advancing glacier, it says the opposite, the advancing glacier covered them.