Where you found it may have something to do with whether it is an artifact or is just natural. You may wish to check whether there was/is a Native American settlement near where you found it. Such rock carving would not be done in a single day, IMHO, or it may be part of a wall similarly adorned in relief if in fact it is an artifact. Sharp edges to the relief may also indicate tools, so see if you can identify tool marks. A museum curator can always give you a definitive answer of course.