As a result of this folding a weak belt was formed along the outer edge of the folded zone. This belt formed the Mesopotamian depression. It has been estimated that the displacement on the Persian side was as much as 9,00o feet, due to the combination of faulting and monoclinal folding. On the west the displacement due to simple faulting alone was slight.
Upper Mesopotamia (Jezirah) contrasts strongly in formation with the lower alluvial valley. The mountain border of the Jezirah begins with the gently folded beds of sandstones, gypsum and con glomerates of Jebel Hamr. There are hills of a similar composition near Mosul on the southwest bank of the Tigris. At this point the river flows along the fault plane between lower Miocene on the southwest and Pliocene Conglomerate on the northwest. Far ther to the northwest the range of hills is capped by basalt. Its continuation Jebel Sinjar also consists of basalt, overlying Mio cene rocks, and the basalts have further a wide extension along the Tigris to the northeast of the Sinjar hills. Further to the west lies the Jebel Abd al-Aziz and still further north of a line drawn from Mardin to Urfah the land rises into the foothills of the Taurus which is composed chiefly of Cretaceous, Eocene and Miocene beds, although there are some basaltic lavas from the old volcano Qara Dagh. In the northwesterly corner of the Mesopo tamian depression between Diarbekr and Mardin gault ammonites have been found but most of the country near Diarbekr consists of volcanic rocks through which the Tigris has cut a 3oo-ft. can yon. The Cretaceous beds to the northwest of Ana are Senonian but below this town they are partly Turonian. Above this belt of Cretaceous rocks along the great western bend of the Euphrates past Birijik and near Samsat there are Miocene beds. To the north they are succeeded by volcanic rocks, through which on both sides of the Taurus the Euphrates has cut a magnificent gorge.
Although lower Mesopotamia consists mainly of alluvium there are extensive outcrops of older formations. There are conglomer ates at Beled on the Tigris, 84 miles from Baghdad by river, and a few miles above Beled at Qadisiyeh and also from Zobeir near Basra to the Euphrates at Suq esh Shuyukh. At Beled the con glomerates rest on a bed of clay.
Conglomerate is however exceptional on the great flood plain and in most of the region the tracts of marshy alluvium are only interrupted by areas of sandy and stony desert. These probably are found where Miocene beds outcrop as low plateaux of gyp siferous marls and are of great interest to students of human history as they probably forkned ancient islands, which may have been inhabited in pre-Sumerian times. One of these plateaux forms a ridge about a mile wide between Museyib and Baghdad. Mio cene outcrops also occur west of Basra.
The most interesting question however in the geology of the alluvial plain, in relation both to the ancient history of Mesopo tamia and certain modern problems, is the growth of the deltaic region. While it seems probable that the old shore line was in
glacial times near Hit the position of the sea coast in Sumerian times is open to considerable conjecture. Langdon in his map in the Cambridge Ancient History puts the site of ancient Eridu actually at the head of the gulf (Eridu lies a few miles south east of Ur), a position fully justified by the explicit statements of ancient texts. Campbell Thompson however in his excavations on this site found fresh water mussels, and suggests that although there is no reason to doubt that Eridu was on the edge of the "sea" that sea was a fresh water lake and not the actual gulf itself but as he also found marine shells it is possible that the fresh water shells were used for food. Sir A. T. Wilson suggests that in ancient times there was a great lake or chain of lakes, all of which except that fed by the Jarrahi poured their waters as at present through the Shatt-al-Arab. He believes that at the be ginning of the Christian era the coast coincided from Bubiyan island with the present shore, curving thence inwards to some where near Abadan, and joining the present coast near Qubban. He suggests further that 90% of the silt deposited in the lakes and therefore never reached the gulf. The size of Bubiyan island shows that in prehistoric times the Euphrates carried great de posits of silt, while the Tigris was less active. Under these circum stances the rapid extension of land at Fao, according to Rawlinson as much as 53 metres per annum between the years 1793 and 1833 is no indication of a stable coast line. There does not appear to be any reason to think that the coast east of Bahmishir has advanced since the Karun abandoned it, nor that the coast at Bubiyan has advanced at all in the past twenty centuries, that is since the Euphrates left that channel. Wilson's conclusions are that the coast line at the mouth of the Shatt-al-Arab advanced slowly till the Karun found its way into that channel at the end of the 18th century. Islands then began to form at the mouth of the Shatt-al-Arab as that stream began to enlarge its bed and to cut the great bend between Mohammerah and Abadan. Wilson suggests that the lacustrine region was formed by silt, sand and gravels brought down from the Arabian plateau during a period of high rainfall in the glacial age by the Batin, at a time when the mouths of the Euphrates and the Tigris were near Hit and Sa marra respectively. This heavy material, together with lighter silt from the Karkhah and Karun have formed the barrier, and the intervening region has only succeeded in becoming more or less filled up in the last few generations.