A block and peg combination for a modular wall construction is disclosed including a cast block of cementations material having a front face, a rear face, two sides with each side extending between an edge of the front face and an edge of the rear face, a bottom, and a top parallel to the bottom. The front face, rear face and two sides are substantially normal to the bottom and top. The front face has a width between the two sides which exceeds the width of the rear face between the two sides whereby the block tapers at the sides from a wide dimension at the front face to a narrower dimension at the rear face for forming curved walls. The bottom defines at least four spaced-apart holes adjacent the respective sides of the block. At least two pegs, each peg having a conical head portion and a shaft portion, are utilized for insertion into the spaced-apart holes on the bottom of the block.