Across the marsh

The warden paused where the boardwalk entered the reed bed. Months of hot weather had warped several planks, lifting their ends above the rest. In the shaded stretches, algae formed a greasy film across the timber. I stopped looking towards the distant hide and watched where the warden placed her boots. Only when she reached the wider platform did I begin to follow.

Select from the passage two words or phrases showing why the writer had to place each foot carefully.