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The close-up of the chalky seam visible in the rock along the left edge of the tunnel is a fault line -- a fracture in the earth's crust where movement occurs.
Although the mountainside appears solid and motionless, faults are a sign that the rocks have experienced shifts and upheavals. The results are apparent all around this area: in the tilted shape of the scenery, the mountains' abrupt rise from the broad Utah Valley, and other fault lines visible along the trail. Combined with seeping water, a fault can also be the starting poin for caverns. Hansen, Middle, and Timpanogos caves dissolved along fractures in the Deseret Limestone.