The Roxbury conglomerate is a heterogeneous formation composed of conglomerate, sandstone, shale, and interbedded volcanic rocks. The coarseness of the conglomerate and the occurrence of tillite in the overlying Squantum formation have suggested considerable relief in the regions from which the sediments were derived. New data indicate that the Roxbury conglomerate was deposited on a surface of high relief. In the Nantasket area the relief was not less than 565 feet, and was presumably at least 845 feet. In the Hingham area the Roxbury conglomerate increases in thickness within less than a mile from 1340 feet in the southern part to 3440 feet or more in the northern part. This is interpreted to mean that in Carboniferous time there was a relief of not less than 2100 feet, with slopes of fourteen degrees or more.