For the next 1800 miles the thin blue ribbon of the Nile, flowing slowly north, unwound over brown soil and green fields, some only a few yards wide, others as broad as an Iowa cornfield.

我理解的句子主干为 the thin blue ribbon of the Nile unwound over brown soil and green fields。 现在分词短语 flowing slowly north作定语。这样对吗？其他的For the next 1800 miles the thin blue ribbon of the Nile是何结构，some... others 在这里又是做什么成分？