Cooper relies on imagery to amplify the fact that Cora and Alice are foils. For example, in Chapter 1, he introduces Alice with light imagery:
The flush which still lingered above the pines in the western sky was not more bright nor delicate than the bloom on her cheek[...]
Alice appears from the very first as a "bright," "delicate," blushing young woman who reflects the light of a beautiful sunset. Her association with light underscores her innocence and softness, and also her whiteness. Cora, on the other hand, is more passionate than her sister. Where Alice defers to authority, Cora questions it. Where Alice repeats racist stereotypes she has heard, Cora challenges them. And where Alice plays the role of the damsel in distress, Cora plays a more active role in determining her own destiny—for instance, she refuses to save her own life by agreeing to marry Magua.
The novel chalks the sisters' differences up to their different mothers. Cora is revealed later in the novel to have distant Black ancestry, whereas Alice's mother was as white as the girls' father. Even before this revelation, Cooper introduces Cora in Chapter 1 with darker imagery that reflects the more violent sides of nature:
The tresses of this lady were shining and black, like the plumage of the raven. Her complexion was not brown, but it rather appeared charged with the color of the rich blood, that seemed ready to burst its bounds.
Cora can pass for white, but according to Cooper there is something more off-putting about her appearance than there is about Alice's. Alice is blond, but Cora's hair looks like the feathers of a scavenger bird. While Alice's flush is a "delicate bloom," Cora's is "the color of rich blood [...] ready to burst its bounds." In the antebellum United States, people could be legally classified as Black (and therefore eligible to be enslaved and otherwise subjugated) if they had a "single drop" of blood from a Black ancestor. Even if Cora's face doesn't immediately appear Black, Cooper's use of imagery makes it sound as though her non-white ancestry is waiting just beneath the surface, ready to "burst" out of her white-passing body.
In Chapter 3, Cooper introduces Hawkeye and Chingachgook. Before he describes the two of them in detail, he uses a barrage of imagery to describe the riverbank where they are resting:
The vast canopy of woods spread itself to the margin of the river overhanging the water, and shadowing its dark current with a deeper hue. The rays of the sun were beginning to grow less fierce, and the intense heat of the day was lessened, as the cooler vapors of the springs and fountains rose above their leafy beds, and rested in the atmosphere.
Cooper begins with visual imagery, describing the way the woods cast shadows over the water. He contrasts the shadow with sunlight and transitions to the sense of physical feeling as he describes the diminishing heat of the sun's rays and the cooling mist that bounces off the water. The reader can imagine what it feels like to breathe in the humid air. Cooper goes on to address yet more senses:
Still that breathing silence, which marks the drowsy sultriness of an American landscape in July, pervaded the secluded spot, interrupted only by the low voices of the men, the occasional and lazy tap of a woodpecker, the discordant cry of some gaudy jay, or a swelling on the ear, from the dull roar of a distant waterfall.
The "breathing silence" evokes the quiet stillness of a humid summer afternoon. Cooper describes the murmuring of voices, the tap of a woodpecker, the cries of a bird, and the rushing of a waterfall that break the silence. This beautiful imagery helps situate all of the reader's senses on the riverbank with Hawkeye and Chingachgook. The pleasure and ease of all these sights, feelings, and sounds also help establish that these two characters are comfortable and happy in the forest. In this novel, the forest is often a very dangerous place. But with this passage, Cooper lets the reader know that it conceals at least as much beauty as danger.
By the end of Chapter 4, Duncan is beginning to mistrust Magua (Le Renard Subtil), but he tries to stay calm as he insists that they stop to eat and rest. Cooper uses sound imagery both to emphasize Duncan's unease and to foreshadow the arrival of Chingachgook, Uncas, and Hawkeye in the nick of time:
“This is well,” continued Heyward; “and Le Renard will have strength and sight to find the path in the morning”; he paused, for sounds like the snapping of a dried stick, and the rustling of leaves, rose from the adjacent bushes, but recollecting himself instantly, he continued—“we must be moving before the sun is seen, or Montcalm may lie in our path, and shut us out from the fortress.”
The "snapping of a dried stick" and the "rustling of leaves" are sounds with which Cooper's readers are almost certainly familiar. They can mean nothing, but they can also signal the presence of someone or something nearby. Duncan is on edge because earlier in the day, Hawkeye and the Mohicans warned him that Magua was not to be trusted. Charged with bringing Alice and Cora to Fort William Henry under Magua's guidance, Duncan does not have much choice in the matter of trusting him. After all, he does not know his way through the woods, especially a way that avoids enemy combatants. Duncan is also a young soldier who trusts his commanders without question, and he has been told that Magua's loyalties lie with the English. Readers who have ever felt spooked by their own suspicions or by their environment will understand the experience of hearing a creepy sound and trying to rationalize it away.
Whereas readers may want Duncan to believe his ears and start looking around for a trap, the sounds in the forest in fact foreshadow a friendly presence. There is indeed a trap lying in wait, but the trap turns out to be for Magua. Chingachgook springs up from the bushes, Hawkeye shoots his gun, and Uncas follows soon after. Magua manages to escape, and he becomes the main antagonist of the novel. But by providing Duncan with surprise backup, Hawkeye and the Mohicans furnish Duncan's escape from the crafty "Renard Subtil." The fact that the noises turn out to foreshadow a friendly presence rather than a sinister one sets the tone for the rest of the novel: the forest is full of hidden surprises, and it is impossible to know if they will be good or bad until they reveal themselves.
In Chapter 9, the band of travelers hides in a cavern. David Gamut uses imagery and a simile to describe the eerie sounds of nature:
“There is melody in the fall of the cataract, and the rushing of many waters is sweet to the senses!” said David, pressing his hand confusedly on his brow. “Is not the air yet filled with shrieks and cries, as though the departed spirits of the damned—”
The waterfall sounds to him like a "melody." He draws on the sense of taste to describe the sound as "sweet," suggesting that the waterfall is so pleasing to the ear that it spills over into other senses. It is disorienting to him to encounter this pleasurable sound when he has lately been listening to the Mohawks' "shrieks and cries" as they pursue the travelers. He uses a simile to compare the sound of all this shrieking to the sound of "the departed spirits of the damned." His ear has been trained to hear the hell-like sound of the Mohawks' battle cries, so it throws him off when instead he encounters the heaven-like sound of the waterfall. Duncan goes on to confirm, impatiently, that the Mohawks have fallen silent for a time. David often seems like a foolish character, but here he demonstrates that of everyone, he is perhaps most in tune with what the sounds of the forest mean. His disorientation at the sudden sound of peace helps emphasize that in the forest, discordant as it might be, danger and peace lie right around the corner from one another.
