What in the second paragraph sets the mood for Heart of Darkness? How would you describe the mood?

In Conrad's "Heart of Darkness," the immediate effect of the second paragraph is one of vastness.  Conrad writes:

The sea-reach of the Thames stretched before us like the beginning of an interminable waterway.

"Interminable" means having or seeming to have no end.  Vastness is before them.  As he looks into the vastness, the narrator sees the sea and sky "welded together" and "the barges drifting up with the tide seemed to stand still in red clusters of canvas...."  A "haze rested on the low shores that ran out to sea in vanishing flatness."  The speaker's eyes are directed outward toward the sea, the vastness, and the air is dark above "Gravesend" (italics mine). Farther back the air is "condensed into a mournful gloom, brooding motionless over the biggest, and the greatest, town on earth."  The yawl is leaving the biggest, greatest town on earth, and going into the vastness of the interminable sea.   

I suggest that the mood is one of vastness and foreboding.  They seem to be leaving the known, and traveling into the unknown.