Which corneal degenerative change produces the characteristic change seen below?

Terrien&;s marginal degeneration is non-ulcerative thinning of the marginal cornea. Clinical features are as follows: 1. Predominantly affects males usually after 40 years of age. 2. Mostly involves superior peripheral cornea. 3. Initial lesion is asymptomatic corneal opacification separated from limbus by a clear zone. 4. The lesion progresses very slowly over many years with thinning and superficial vascularization. Dense yellowish white deposits may be seen at the sharp leading edge. Patient experiences irritation and defective vision (due to astigmatism). Complications such as perforation (due to mild trauma) and pseudopterygia may develop. Treatment is non-specific. In severe thinning, a patch of the corneal graft may be required. Ref: Khurana; 4th edition; Pg.117