What are some examples of personification and Maya Angelou's ideas of the past and future in "On the Pulse of Morning"?

In this poem, Maya Angelou uses the images of rock, river, and tree to describe the United States. She talks about the troubled past of this land, on which mastodons and dinosaurs have left their bones ("dried tokens") and where, more recently, humans have left troubling remains. Though Angelou says we are just a little lower than angels, we have lived in "ignorance" and left "debris." The country was settled by people "desperate for gain" who drove the Native Americans off the land and enslaved others. The history of this country has been full of "wrenching pain." 
Although the past has been troubled and imperfect, a "nightmare" for some, Angelou holds out great hope for the future. She invites all Americans, rich or poor, straight or gay, Muslim, Christian, or Jewish, to join in building a better world, for all are "yearning to respond" to the call of this land. We can all give birth to the American dream again. We can "shape it" and "sculpt it." "Each hour holds new chances," she writes. We should meet each other with hope, look into each other's eyes (by which she means really see each other) and come together with a "good morning" greeting that will symbolize the dawn of a new and better America. (It's useful to note the poem is an occasional poem, written for President Bill Clinton's inauguration, and so reflects the hope he wanted to project.) Her hope for the future is that, although different, Americans can come together in community and solidarity. 
Angelou personifies geographic features of America by giving them human characteristics. A rock "cries out" to us with advice, as a human might. Likewise, a tree speaks to us and a river sings a "beautiful song" and also speaks to us as a human would, saying, "Come, rest here by my side." The horizon leans forward to speak to us—as a person would in conversation—and the day has a "pulse" like a human pulse, as if it is alive, with a beating heart and blood flowing through its veins.