What do the shadows represent?

The rock itself represents Quebec, in which the story is set. It is a haven of relative stability and security for French settlers in the still wild and generally unspoiled wilderness of North America. Nevertheless, life for settlers such as Cécile and her father is still incredibly hard in this part of the world, as can be seen from the harsh, cold winters they are forced to endure. The inclement weather is just one of many "shadows," or negative aspects of life in Quebec that make living there such a struggle at times.
But in due course Cécile comes to accept the natural rhythms of life in the New World. Far from being a source of privation and hardship, nature becomes a place of delight and discovery. Cécile attunes herself to her new environment, adapting to each individual season as it arrives, come rain, hale, or shine. Life will still be full of hardships for her, full of "shadows," but she will remain connected to the soil, becoming a feature of the landscape every bit as much as the forests, the rivers, and the rocks.