The Apennines to the east of this pass have an average height of 3,300 feet, and beyond the Pass of Giovi (1,538 feet), through which the road leads from Genoa to the northern plains, many summits attain a height of 4,500 feet. Several spurs, abounding in ravines, extend here to the north. The main chain, at the same time, retires from the coast, and the Pass of Pontremoli, which separates the Ligurian from the Tuscan Apennines, and through which leads the road from Parma to Spezia, is no less than thirty miles from the sea. In this eastern portion of the Genoese Apennines a spur detaches itself from the main chain, and terminates in the fine promontory of Porto Venere, a magnificent rock of black marble, surmounted formerly by a temple of Venus. This spur, which protects the Gulf of Spezia against westerly winds, has at all times constituted an obstacle to the intercourse between neighbouring peoples, not so much on account of its height, but because of its steepness. In some places the crest of the Apennines is hardly more than four miles from the sea. The slope, in such places, is exceedingly steep, and roads can ascend it only in numerous windings.[77] The small width of the maritime slope of the Ligurian Apennines accounts for the absence of perennial rivers. The most considerable streams to the east of the Roya, which runs for the greater part through French territory, such as the Taggia or the Centa, only assume the appearance of rivers when the snows melt, or after heavy rains. Ordinarily they are but small streams, closed at the mouth by bars of pebbles. Between Albenga and Spezia, for a distance of 160 miles, there are only torrents, and in order to meet again with a real river we must go beyond the Gulf of Spezia. This river is the Magra, which separates Liguria from Etruria, and which, up to the epoch of Augustus, formed the boundary of Italy. Its alluvium has converted an ancient bay of the sea into a lake, and formed a beach, 1,300 yards in width, in front of the ancient Tyrrhenian city of Luni, which formerly stood on the seashore. The want of great rivers in Liguria is compensated for to some extent by subterranean water-courses. Several springs rise from the bottom of the sea, at some distance from the shore. The springs of La Polla, in the Gulf of Spezia, are amongst the most bountiful amongst them. They have been isolated by the Italian Government from the surrounding salt water, and their water is supplied to ships.