They may have been skating

—according to the following definition of Slide:
"I. intr. 1. To pass from one place or point to another with a smooth, continuous movement, esp. through the air or water or along a surface.

"b. To move in this manner while standing more or less erect upon a surface, esp. that of ice.

"Formerly used of skating, now distinguished from it." (O.E.D.)

Or sliding
            —perhaps in the manner of the Pickwickians.