But nature has set a permanent mark, half way down the Mesopotamian lowland, by a difference of geological structure, which is very conspicuous.

Near Hit on the Euphrates, and a little below Samarah on the Tigris, the traveller who descends the streams, bids adieu to a somewhat waving and slightly elevated plain of secondary formation, and enters on the dead flat and low level of the mere alluvium.

The line thus formed is marked and invariable; it constitutes the only natural division between the upper and lower portions of the valley; and both probability and history point to it as the actual boundary between Chaldaea and her northern neighbor.
The extent of ancient Chaldaea is, even after we have fixed its boundaries, a question of some difficulty.

From the edge of the alluvium a little below Hit, to the present coast of the Persian Gulf at the mouth of the Shat-el-Arab, is a distance of above 430 miles; while from the western shore of the Bahr-i-Nedjif to the Tigris at Serut is a direct distance of 185 miles.

The present area of the alluvium west of the Tigris and the Shat-el-Arab maybe estimated at about 30,000 square miles.
But the extent of ancient Chaldaea can scarcely have been so great.