In "Ozymandias," how does the quote "boundless and bare" show the power of nature?

The poem "Ozymandias" by Percy Bysshe Shelley ends with the following lines:

Round the decay
Of that colossal Wreck, boundless and bare
The lone and level sands stretch far away.

The poem describes a narrator meeting a traveler who recounts seeing the ruins of a statue of "Ozymandias," a name ancient Greek and nineteenth-century British writers used for the Egyptian pharaoh Ramesses II. The sculpture itself is a great and monumental work. Ramesses II was one of the most powerful and feared men of his period, ruling a great and prosperous kingdom, and renowned for his military conquests. Despite the statue being part of a magnificent temple in a wealthy and highly developed nation for its period, all that now remains of it are fallen fragments poking out from the sand.
The final lines of the poem, in describing the scene as "boundless and bare," suggest that nature, in the form of the desert sand, is so powerful that it has almost completely covered up evidence of this great and powerful civilization. This suggests that no matter how powerful a king or a kingdom, in the end, nature is even stronger and more powerful and eventually will, as it were, conquer the conquerors.