Hawksbill station

In the mid-21st century, time travel is used to send political prisoners to Hawksbill Station, a prison camp in the late Cambrian Era. When the latest arrival suspiciously deflects questions about his crimes and knowledge of 'Up Front', the inmates decide to find out his secret. (This story was used as the basis of the novel "Hawksbill Station".) Nebula Award(R) Nominee, Hugo Award Nominee