Becky Sharp’s “pic-nic” begins with “a giddy and rollicking company” on an island in the Mississippi. After traveling by old ferryboat three miles below the town of Hannibal, the picnickers “swarmed ashore and soon the forest distances and craggy heights echoed far and near with shoutings and laughter.”
The picnic turns adventuresome when Tom and Becky get lost in McDougal’s Cave, a series of caverns along the shore. It’s fun until Tom and Becky lose their way. Though Becky wanted to be alone with Tom all along, being lost at a picnic is not what she wants. It’s dark and here are bats. They are lost and she is frightened. Becky whimpers, “Tom, Tom, we’re lost! We’re lost! We never can get out of this awful place! Oh, why did we ever leave the others!” With bravado, mostly feigned, Tom assumes the role of the gallant knight, while Becky plays damsel in distress. To stave off hunger, Tom offers to share what food there is – cake: “Tom, I’m so hungry!” Tom took something out of his pocket. “Do you remember this,” said he. Becky almost smiled. “It’s our wedding cake, Tom.” “Yes—I wish it was as big as a barrel, for it’s all we’ve got.” “I saved it from the pic-nic for us to dream on Tom, the way grown up people do with wedding cake—but it’ll be our–” She dropped the sentence where it was. Tom divided the cake and Becky ate with good appetite, while Tom nibbled at his moiety. There was abundance of cold water to finish the feast with.”
True W. Williams. The Wedding Cake.
True W. Williams was Twain’s choice to illustrate The Adventures of Tom Sawyer and gave free reign. Writing to his friend William Dean Howells, Twain confided “Williams has made about 300 rattling pictures for it— some of them very dainty. Poor devil, what a genius he has, and how he does murder it with rum. He takes a book of mine, and without suggestion from anybody builds no end of pictures just from his reading of it.” Others have illustrated the caves episode, particularly Norman Rockwell. 
See Mark Twain-Howells Letters, edited by Henry Nash Smith and William M. Gibson, (Cambridge: Belknap Press, 1960); Don Taylor. Tom Sawyer (1973). Screenplay by Richard M. Sherman and Robert B. Sherman based on Mark Twain’s novel (1876); See True Williams. ; Beverly R. David, Beverly R. Mark Twain and His Illustrators: (1875-1883). Vol. 2 (Troy, New York: Whitston Publishing Company, 2001; Barbara Schmidt. A Closer Look at the Lives of True Williams and Alexander Belford, (2001) http://www.twainquotes.com/TWW/TWW.html