The Holes Around Mars

"Is there any possibility," asked Janus, as we walked, "that it could be a natural phenomenon?"

"There are no straight lines in nature," Randolph said, a little shortly. "That goes for a bunch of circles in a straight line. And for perfect circles, too."

"A planet is a circle," objected Janus.

"An oblate spheroid," Allenby corrected.

"A planet's orbit--"

"An ellipse."

Janus walked a few steps, frowning. Then he said, "I remember reading that there is something darned near a perfect circle in nature." He paused a moment. "Potholes." And he looked at me, as mineralogist, to corroborate.

"What kind of potholes?" I asked cautiously. "Do you mean where part of a limestone deposit has dissol--"

"No. I once read that when a glacier passes over a hard rock that's lying on some softer rock, it grinds t