The Eagle Rock originally had a hop in it but as it spread north and south it was discarded in favor of a shuffle. The Eagle Rock started to wane in popularity in the early 1920s.
Other stories on it creation is said to be The 'Eagle Rock Reservation', (1920s) in Northern NJ where the Indian dancers would outstretch their arms like an eagle and sway back and forth. Another is the 'Eagle Rock Ferry of the Snake River Valley in Idaho during the 1860's is said that 'Rock Island' had an eagles nest on it, again the Eagle would sway back and forth (or rock) and the Ferry boat dancers would imitate the bird.
The Eagle Rock replaced the similar dance called the 'Buzzard Lope' in popularity as the buzzard lope was associated with plantation life and considered too risque?. Another dance called the 'Shout' was similar to the Eagle Rock as well. Most people today would not be familiar with this dance step if not for a little dance song called "Ballin' the Jack" which uses the Eagle Rock as part of the lyrics and the dance itself. Danny Kaye is probably most remembered for this song.
Basic Step as it might be applied to another dance: (below Lyrics from 'Ballin The Jack' sheet music):
First you put your knees together, close up tight,
Then you sway 'em to left , then you sway 'em to the right. (a modified Georgia Grind)
Step around the floor kind of nice and light, (implies a Cat Walk?)
Then you twis' around, and twis' around with all your might, (a hip / Pelvis Roll)
Style and Grace, Swing your foot way round, then bring it back,
Now that's what I call Ball-in the jack.
Then you do the Eagle Rock with style and grace, (Raise arms Like an eagle)
Swing your foot way'round then bring it back,
Now that's what I call Ballin' the jack"...