Sentence:  The day was cool and snappy for August, and the Rise all green with a lavish nature. Now we <WSD>plunged</WSD> into a deep shade with the boughs lacing each other overhead, and crossed dainty, rustic bridges over the cold trout-streams, the boards giving back the clatter of our horses' feet:.... 
Meaning of the word marked with <WSD> tags:  To fall or rush headlong into some thing, action, state or condition.