Beside the disused track

The surveyor asked me to stay beside her as we followed the disused track. The crushed stone shifted beneath our boots, and the old sleepers stood at irregular heights rather than lying level. We were concentrating so closely on the ground that neither of us noticed the station building until its shadow fell across us. There, at last, we stepped on to a firm platform.

Select two words or phrases from the passage that show the ground was difficult to walk over safely.