The Croods

A cave family called the Croods survives due to the overprotective nature of their stubborn, stern patriarch, Grug. The only one who questions the family's sheltered life is his teenaged daughter Eep who frequently disobeys her father's orders out of curiosity, which he finds dangerous. Grug and Eep, along with her mother and his wife Ugga, her grandmother Gran, and her younger brother and sister Thunk and Sandy face time sheltered in their cave home.
Eep sneaks out when she sees what she discovers to be a torch of fire and she encounters an inventive modern human boy named Guy and his pet sloth Belt. He warns her of an impending apocalypse and offers to take her with him, but concerned for her family, Eep stays, getting a shell horn from him to blow in case she needs his help. Reuniting with her frantic father, she tries to tell her family what Guy told her, but fearing things that are "different" and "new" they destroy her horn.
A massive earthquake then destroys their home, and to avoid the earlier carnivore, they descend down into a tropical forest that lay hidden behind their cave all the time. Encountering a "Macawnivore" a brightly colored feline that Gran dubs "Chunky the Death Cat," the family flees him until he is deterred by swarms of piranhakeets that devoured a ground whale. Using another horn, Eep calls to Guy who rescues them from the birds with his fire. After a great deal of confusion regarding their first contact with fire, Grug imprisons Guy in a log until he can guide them somewhere safe.
Outrunning the destruction, Guy is trusted enough to be let out of the log and he gives the Croods rudimentary shoes to walk over the harsh landscape as he leads them to a mountain in which he says will be safe. Guy also tells them stories of "Tomorrow" a haven of safety where he is headed and where curiosity is not deadly as Grug had claimed. At his treetop home, Grug sees the impression Guy is leaving on his family and he becomes jealous, especially when he realizes that Eep has fallen in love with Guy. Attempting to invent things like Guy, Grug only further embarrasses himself and drives his family further away from him. After the family is split up in a labyrinth of tunnels, all but Grug manages to escape by coming up with ideas of overcoming obstacles in their paths.
Reaching the mountain, Grug tries to force his family to hide out in a cave, but they resist, telling him that they can't live in caves anymore, that they don't want to "survive" but to "live". This enrages Grug, who attacks Guy and the both of them end up in a tar flow where Grug learns that Guy's family had perished in one. Realizing that Guy's method of survival is better for his family, Grug works with him and they lure Chunky into a trap to free themselves. The family reunites, then flees a massive cataclysm as the land begins to violently rip apart.
The family is cut off from their destination by a continental split, but Grug, realizing the errors of his ways decides to throw his family to safety. He shares an invention he calls a "hug" with Eep, briefly before sending her across as well. Cut off from his family, Grug finds a cave for safety, where he encounters Chunky; who is truly a frightened and sweet feline, and he comes up with an idea to get across the chasm. Using a skeleton and the Piranhakeets to fashion a simple airship, Grug manages to send themselves; including several animals the family had encountered during their journey, across the chasm, reuniting with his family once again. He apologizes to them all and promises to never be so overbearing again.
Later, the Croods now live on the land and have settled on a vast beach where every day they can follow the light to "Tomorrow."

Question: Who imprisons Guy in a log?
Answer: Grug