A walker in the desert feels himself shrinking as the light fades. "I turn thin" is a striking way to put it: the speaker seems to lose substance, as if the vast, emptying landscape wears him down to almost nothing. It suggests smallness, exposure, and perhaps a quiet unease.
Then night arrives, and the poem gives it three shapes instead of one explanation. A horse suggests something dark, powerful, and moving toward him. A cross brings to mind the shape of the horizon or a figure with outstretched arms, and carries a hint of death or sacrifice. The last image, "the day's black dress," is the most intimate: night becomes something the day puts on, like mourning clothes. Together the images make dusk feel both beautiful and solemn. The mood is hushed and a little eerie, a moment of awe where the speaker is humbled by the coming dark rather than simply afraid of it.