A construction block includes rectangular front and rear panels and at least one web connecting the front and rear panels. Each of the front and rear panels has top, bottom, and right and left edge surfaces. At least one web having top and bottom surfaces co-extensive with the top and bottom of the front and rear panels is provided to define top and bottom surfaces of the block. The top surface of the block has a shallow rectangular ridge along its length, flanked by outer support surfaces. The lower surface of the block has a shallow rectangular channel along its length, flanked by flat outer feet corresponding to the outer support surfaces. The right and left edge surfaces of the front and rear panels are profiled to interfit with one another.