Union soldiers gathered these rocks into defensive barricades on Little Round Top after repulsing repeated Rebel attacks up the slopes on July 2nd. That day, their protection was hastily assembled piles of tree trunks and branches. The Union soldiers upgraded to rocks in expectation of further Confederate assaults the following day. That never happened. The Rebs were instead busy attacking the center of the Federal line to the north in the disastrous Pickett's Charge.