It's not a conventional Turtle.
Below is a very simplistic drawing of how a conventional turtle is constructed. There is a support rock under the rock that forms the head and neck, and a counterbalance above and behind that. The legs and tail are not required, but often there. More often than not, the body of the turtle was composed of many rocks that were cut to roughly form the shape of the shell.
While that is what a conventional turtle looks like, it's not the only way they were made. I found one that looks a little like yours, but you would have to see a man made marking or monument somewhere in the direction your turtle head is facing.
The bad thing is, because the head is cut off, that is usually a sign that whatever the turtle is a monument for is gone. HOWEVER, a lot of times, if someone before you has seen it, and couldn't find what it pointed to, they would cut the head off themselves to deter other people from looking any further.
Where in the country is that? Remember, the Spanish (and Amerindians) were the only ones to use the turtle as a marker. Neither the French nor the English used this sign.
Best,
Mike