Abstract Mesas and buttes of the central Colorado Piedmont are composed of at least twodistinct rock types, which differ in their cohesiveness and resistance to erosion. Thelower parts of the exposed stratigraphic section are poorly cemented, Upper Cretaceousto Middle Eocene sandstones of the Dawson Formation. The caprocks arecomposed of one or more resistant formations of Late Eocene age: the Castle RockConglomerate, Wall Mountain Tuff, and the conglomerate of Larkspur Butte. Theseformations were originally deposited in topographic lows, but due to their resistance,they now cap prominent buttes and mesas of the Colorado Piedmont. Erosion of thecaprock through progressive retreat of the butte scarp produces colluvium that has ahigher resistance to erosion than the poorly cemented underlying sandstone. Once the caprock of a butte has been removed by erosion, the underlying weaklycemented Dawson Formation is readily eroded. Ultimately, the armored lower slopesof the former butte remain as a circular ridge standing as much as 100 m above thesurrounding topography. This process produces a topographic low surrounded byrelict faceted slopes where the flat top of the butte once stood. Prominent alluvial fans are associated with some of these annular features, andthey record the main phases of butte removal and excavation of the central part of thearmored slopes. Multiple generations of alluvial fans contain coarse- and fine-grainedfacies that represent changes in effective stream power and record alternating phasesof aggradation and erosion. The degree of soil development in the fan alluvium andheight of the fan surfaces above streams indicates the oldest preserved gravel fandeposit is of late-middle Pleistocene age. The youngest luminescence (optically stimulatedluminescence) dated alluvial fans were deposited during the late Pleistoceneabout the time of the Pinedale glacial maximum in Colorado, ca. 21,000 yr B.P. Keywords: Colorado Piedmont, talus flatiron, talus flatiron ring, inverted topography.

Abstract Spanish Quaternary sediments and landforms record glacial, alluvial, fluvial, lacustrine, aeolian, coastal and volcanic environments. Within these environments numerous processes acted on different lithologies and structures during changing climate, neotectonic activity, and anthropogenic influence. Consequently, the current landscape of Spain comprises a complex palimpsest of inherited and active landforms. Spain has a mountainous relief, with an average height of 660 m for the whole Iberian peninsula. This high relief is related to the presence of extensive plateaux surrounded by mountain ranges (Cantabrian mountains, Pyrenees, Catalonian Coastal Ranges, Iberian Ranges and Betic Cordillera). The highest elevation in the Iberian peninsula is found in the Betic Cordillera (Mulhacén, 3481 m), although in the Pyrenees there are many peaks above 3000 m. The hydrographic network of the Iberian peninsula has a main divide separating the basins draining to the Mediterranean sea from those draining into the Atlantic ocean. Most of the major rivers drain to the Atlantic and run for a significant part of their course through Tertiary depressions such as those of the Duero, Tajo and Guadalquivir basins. The Ebro river is the main Mediterranean fluvial system and flows through the Ebro depression. These Iberian rivers drain a landscape affected by contrasting climatic belts, with a humid zone in the north, a semi-arid zone situated in the SE and in the large Tertiary depressions, and semi-humid conditions elsewhere. Chronostratigraphic scale for the Mediterranean and Atlantic (modified after Bardají 1999 ). Palaeomagnetic scale after Cande and Kent (1995) . Isotopic scale