Finding the survey plot

At the edge of the pine plantation, the previous team had marked a square survey plot with wooden pegs. By morning, snow lay level across the clearing, swallowing the low marker pegs. The notes placed the northern corner beside a forked birch, yet a cluster of young birches stood there, each dividing into two pale trunks. I brushed snow from several roots before checking the compass and pacing the distance again.

Select two words or phrases from the passage that show why the boundary of the survey plot was difficult to locate.