Red sediments of Tertiary age crop out alongside the southern border of the Cantabrian Mountains in the northern part of the Duero basin. They consist mainly of conglomerates with quartzite pebbles, sandstones, and sandy, loamy, and marly deposits, all with a deep red colour. Detailed analyses were made on grain size composition, on pebble roundness, and on sand grain roundness and sphericity. The results are presented in triangle-diagrams for nomenclature, cumulative curves of size frequency distributions, graphs showing changes of sediment properties with transport distance, and in a facies map. The following conclusions can be drawn: (1) the source area of the sediments was a mountain chain with outcropping Paleozoic and Mesozoic deposits and their weathering products; (2) the transport of the debris occurred by rivers, which flowed in a south-easterly direction; (3) the deposition took place in the mountain foreland, the coarse sediments being deposited nearer to the mountain area than the finer ones; (4) the transport length was fairly short; (5) the conglomerates exposed in the source area provided rounded pebbles to the gravelly sediments deposited in the basins (6) the rivers left the mountain area at the same sites as the present ones. Finally the description of two type locality sections gives an impression of the red bed lithology.