Monument Valley (Arizona/Utah)

Yesterday we drove from Williams, Arizona to Moab, Utah - a long drive. Odd at times for more reasons than one. We drove through a couple Reservations and noticed horses or cows alongside the road. One of the horses looked very gaunt while 2 of the cows on the roadside, miles apart, were dead. Sad.
The other odd thing though is as the photos show.

Mexican Hat is the name of this formation.

It makes no sense - no explanation as to where it came from; was it left behind and the others around it disintegrated? I'll try to remember the question when we hit the visitor center today. Today we're off to do Arches National Park.