[131] The Paso Paraguayo, which has cost the Argentine hydrographic service most work, did not exist at the middle of the nineteenth century. It seems that the channel then kept to the cliff as far as Benavidez, and was continued as far as the source of the Parana Pavon by a very pronounced buckle, of which the Monriel lagoon is a scar. In 1895 the Paso was only fifteen feet deep. The delta begins at San Pedro. The Parana Guazu, or main arm, leaves the cliff on the right bank and passes to the Uruguayan bank opposite Carmelo. The Parana de las Palmas, which branches off from it to the south and passes before Campana and Zarate at the foot of the tableland, is deep and easy to navigate, but it is closed at the bottom of the estuary by a six-foot bar, which makes it a sort of blind alley opened only above. The arms of the zone of the delta differ from those of the river-zone proper in the irregularity of their course. Flowing between long islands, they sometimes lie in straight stretches and at other times in meanders or almost perfect buckles. The channels of the southern part of the delta, near Buenos Aires, are called _caracoles_ (snails) on account of their winding shape. The weakness of the current, which is held up by the tide, is seen also in the distribution of the greater depths; they are no longer uniformly found along the concave edge of the bends, but are scattered irregularly. On the Parana Guazu a depth of 130 feet has been ascertained. Its minimum depth is twenty-two feet. [Illustration: THE PARANA ABOVE THE ESTUARY. _Right bank. The river has moved away from the_ barranca, _leaving at its foot an alluvial plain of imperfect spiral form_. PLATE XXII. To face p. 250.] The study of the estuary may be taken separately from that of the river. It consists of three parts, unequal in size, which open with increasing breadth toward the Atlantic. The upper Rio de la Plata, above Colonia and Punta Lara, has a width of about thirty-five miles. The middle Plata, twice as wide, extends to the latitude of Montevideo and Punta de las Piedras. Then the outer harbour opens between Maldonado and Punta Rasa. The water is still fresh in the middle estuary up to eighty miles below Buenos Aires. The bottom is alluvial except in the channels between Martin Garcia and Colonia.[132] Differently from up the river, where the channels have sandy bottoms, while the banks are of fine clay, the channels of the estuary have bottoms of mud and clay. In the outer harbour the pilots recognize the approach of banks by the sand which is brought up by the sounding-lead. The action of the waves, which is not found in the river, accumulates stuff of comparatively large size and weight on the banks. [132] The granite which outcrops at Martin Garcia also forms the platform of the English Bank in the outer harbour.