We understand from the passage that at some point during the 200,000 years during which the tracks lay hidden, ----.

After a heavy rain one day 200,000 years ago,
someone small walked across some sand. Sand
covered the tracks and eventually they hardened into
sandstone. More centuries passed, and the
sandstone eroded. Some construction workers on a
break in Nahoon, South Africa, discovered the
tracks, which were thought to be 30,000 years old.
Now geologist Dave Roberts, at the Council for
Geoscience in Cape Town, has redated the
impressions and says they are 200,000 years old,
the oldest human footprints on Earth. "We have far
more powerful dating techniques now", says
Roberts. He used thermoluminescence, a dating
method that measures when sand grains were last
exposed to light. Only about seven inches long, the
prints clearly show five toes and a well-developed
arch.