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!
Slab City, also called The Slabs, is a largely snowbird community in the Sonoran Desert located in Imperial County, California, 100 miles northeast of San Diego and 169 miles southeast of Los Angeles within the California Badlands, and used by recreational vehicle owners and squatters from across North America
