It may be that by Darius's time a natural waterway passage which had existed between the Heroopolite Gulf and the Red Sea in the vicinity of the Egyptian town of Shaluf (alt. "Chalouf" or "Shaloof"), located just south of the Great Bitter Lake, had become so blocked with silt that Darius needed to clear it out so as to allow navigation once again. According to Herodotus, Darius's canal was wide enough that two triremes could pass each other with oars extended, and required four days to traverse. Darius commemorated his achievement with a number of granite stelae that he set up on the Nile bank, including one near Kabret, and a further one a few miles north of Suez. The Darius Inscriptions read:  The canal left the Nile at Bubastis. An inscription on a pillar at Pithom records that in 270 or 269 BCE, it was again reopened, by Ptolemy II Philadelphus. In Arsinoe, Ptolemy constructed a navigable lock, with sluices, at the Heroopolite Gulf of the Red Sea, which allowed the passage of vessels but prevented salt water from the Red Sea from mingling with the fresh water in the canal. The Red Sea is believed by some historians to have gradually receded over the centuries, its coastline slowly moving southward away from Lake Timsah and the Great Bitter Lake. Coupled with persistent accumulations of Nile silt, maintenance and repair of Ptolemy's canal became increasingly cumbersome over each passing century. Two hundred years after the construction of Ptolemy's canal, Cleopatra seems to have had no west–east waterway passage, because the Pelusiac branch of the Nile, which fed Ptolemy's west–east canal, had by that time dwindled, being choked with silt. By the 8th century, a navigable canal existed between Old Cairo and the Red Sea, but accounts vary as to who ordered its construction—either Trajan or 'Amr ibn al-'As, or Omar the Great. This canal was reportedly linked to the River Nile at Old Cairo and ended near modern Suez. A geography treatise by Dicuil reports a conversation with an English monk, Fidelis, who had sailed on the canal from the Nile to the Red Sea during a pilgrimage to the Holy Land in the first half of the 8th century  The Abbasid Caliph al-Mansur is said to have ordered this canal closed in 767 to prevent supplies from reaching Arabian detractors.