"Riding in advance, we passed over one of these great plains; we looked back and saw the line of scattered horsemen stretching for a mile or more; and far in the rear against the horizon, the white wagons creeping slowly along"

Francis Parkman's quote provides a vibrant picture of a journey across the expansive Fantastic Plains, catching both the grandeur and the struggle fundamental in such an undertaking. The imagery paints a scene of movement and development set against a huge, open landscape-- a hallmark of the American West during the time of westward growth.

The expression "Riding ahead of time" suggests a position of leadership or exploration, indicating that the narrator is at the forefront of this exploration, both physically and perhaps symbolically. This indicates a sense of experience and discovery, where those at the head are tasked with navigating brand-new surfaces and potential obstacles.

The "excellent plains" describe the expansive flatlands that are iconic to the central United States. By explaining them as "terrific", Parkman might be highlighting both their physical size and their significance within the American landscape, embodying both opportunity and a challenging vastness that settlers need to traverse.

When Parkman explains looking back and seeing "the line of spread horsemen stretching for a mile or more", he catches the scale of the exploration while likewise meaning a degree of disorganization or vulnerability. The term "scattered" recommends that these people, though part of a larger collective, are expanded, possibly separated in their experience of this vastness, highlighting the personal courage and durability needed in pioneering such a journey.

Finally, the "white wagons sneaking slowly along" creates a striking visual contrast against the plains and stresses the arduous nature of the journey. The slow motion of the wagons represents the challenges of the surface and the large effort needed to progress. It recommends a determined determination, as these travelers inch toward an unsure future.

Overall, this quote encapsulates the spirit of exploration, the troubles of the journey, and the common yet individualistic efforts of those who ventured westward. It stimulates a sense of nostalgia and regard for the endeavor of crossing a land that was as treacherous as it was promising.