Before glaciation, mountain valleys have a characteristic "V" shape, produced by eroding water. During glaciation, these valleys are widened, deepened, and smoothed, forming a "U"-shaped glacial valley. The erosion that creates glacial valleys eliminates the spurs of earth that extend across mountain valleys, creating triangular cliffs called truncated spurs. Within glacial valleys, depressions created by plucking and abrasion can be filled by lakes, called paternoster lakes. If a glacial valley runs into a large body of water, it forms a fjord.

What shape do mountain valleys have pre-glacation?