In addition to the large shield volcanoes, Tharsis contains a number of smaller volcanoes called tholi and paterae. The tholi are dome-shaped edifices with flanks that are much steeper than the larger Tharsis shields. Their central calderas are also quite large in proportion to their base diameters. The density of impact craters on many of the tholi indicate they are older than the large shields, having formed between late Noachian and early Hesperian times. Ceraunius Tholus and Uranius Tholus have densely channeled flanks, suggesting that the flank surfaces are made up of easily erodible material, such as ash. The age and morphology of the tholi provide strong evidence that the tholi represent the summits of old shield volcanoes that have been largely buried by great thicknesses of younger lava flows. By one estimate the Tharsis tholi may be buried by up to 4 km of lava.