Why do two ice blocks join to form one block when pressed together

A Block of Ice

On a smooth table two blocks of masses 2.5kg and 1.5kg are placed one over the other as shown in figure. If the coefficient of static friction between two blocks is 0.2, the maximum horizontal force to be applied on the lower block so that the two blocks move together is (g = ms^(-2) )

Two ice cubes are pressed over each other until they unite to form one block. Which of the following forces dominates for holding them together?

Two ice cubes are pressed over each other until they unite to form one block. The force mainly responsible for holding them together is