Pretty sure it’s the frontier
İam pretty decent. Which year group are you in??
Thoreau mostly uses imagery to illustrate time in the excerpt. The second sentence is saying that he is in the midst of time/living, and he can "detect how shallow it is," meaning that he is aware that life is short and that the end is inevitable. Thoreau also describes time as being fleeting, but ever-present.
I hope this helps!
Write about what could’ve happened if he was never born or if you were able to go back in time and stop his birth