The lowlands dell’avampaese Carpathian differ profoundly from the Transylvanian Basin: the accumulation forms and appearance of the floodplain are best preserved, especially in the southern part corresponding to the ancient Wallachia, where 2 / 3 of the surface are below 200 m., and where the outcrops of Neogenic strata are, at least to the east of the Olt, almost unknown. The wrinkle does not appear to have gone beyond the very edge of the Carpathian arch; and also to the west of the Olt the hills that extend almost as far as Craiova have been engraved by erosion in the sensitively horizontal clayey-sandy layers of the Neogenic period.
The old name of Oltenia indicates the western countries, more rugged: that of Muntenia designates the eastern region, where the plains predominate, but where the only populated area, and therefore important in the past, was for a long time the contact strip. with the sub-Carpathian hills.
In Oltenia, the edge of the Carpazî is marked by a succession of sub-Carpathian depressions, of which Târgu-Jiu is the center, an area of constipation in which a whole series of alluvial terraces occur. Only towards the outlet of the Olt are the strata straightened and form asymmetrical ridges that reach several hundreds of meters in relative height. In the far west the conditions are, once again, different and the erosion is just beginning to highlight the contact between the Neogenic and the Carpathian folds, leveled during the Miocene, which form the plateau of Mehedinţi, furrowed by gorges wild in the limestone bands.
South of a line drawn roughly from Turnu-Severin to Craiova and Slatina, the Neogenic hills disappear and the large flat-bottomed valleys that cut them merge into an alluvial plain covered with löss, which slopes down to the shore of the Danube.
The plain of Muntenia is notable for the layout of the hydrographic network. The rivers, instead of descending directly to the Danube, undergo an inflection towards the left which is all the more accentuated the more one proceeds towards the east, so as to pass from the south-east to the east and finally (Buzău) to the north-east. The influence of a subsidence movement is noted, the main center of which would be towards Focşani-Brăila, in the region where the remnants of the Cimmerian range mentioned above abruptly disappear, and the folds of the high sub-Carpathian hills advance further at the expense of the plain. All the rivers of eastern Muntenia have moved to the left, abandoning terraces, such as those that form a fan south of Piteşti, or the marshy plains indicating the primitive southward courses of Dâmboviţa and Ialomiţa north of Bucharest, or that which was followed by Buzău east of Făurei. The drainage of the low plains of the Siret and its rivers (Putna, Râmnic and Buzău) is difficult.
The Danube valley forms by itself a true marshy region, cluttered with lakes and dead arms, of a width that progressively increases until it reaches 10 to 12 km. in the Balta of Brăila. Its course, in the section in which it is directed from west to east, was determined by the sinking of the pre-Balcanic Cretaceous plateau, which rises to a peak, as can be observed in the southern Dobruja quadrilateral; while in the section directed from south to north it was determined by faults that limit the Dobruja pillar and the sinking area of the eastern Muntenia. In both cases the river was pushed back to the right due to the influence of the floods brought by its Carpathian tributaries. Near Galati it touches the spurs of the reliefs of the old Cimmerian range.
No doubt the waters of the Black Sea penetrated through a gulf to Brăila in the Quaternary period. The lakes that formed at the confluence of the tributaries from Moldavia and Bessarabia represent what remains of the unfilled valley bottoms that reconnected to the Danube valley. Recent floods have caused the sea to recede, thus forming the Danube delta.
This delta is notable for its unfinished character: it seems to progress with such rapidity that the mainland between the arms of the river does not have time to definitively consolidate: in the north and south, vast lagoons are isolated from the sea by coastal strips (Sinoe and Razelm lakes in Dobruja, Bessarabia lagoons).
The surface of the delta itself, between the S. Giorgio arm to the south and the Chilia arm to the north, is covered with water during almost the whole year, with the exception of the natural dams (grindu), between which the arms flow, and the ancient coastal ridges surmounted by duria, which mark the successive positions of the delta front. The main mass of the waters of the Danube currently passes through the Chilia arm and the abundance of floods causes the development of a small secondary delta, which threatens to compromise the outlet of the Sulina arm, which the International Danube Commission has arranged for navigation (see danube).