In spite of the conclusions embodied in the nautical instructions, which describe the estuary as a theatre of rapid changes "occasioned by the continual deposits of sand brought down by the Parana and the Uruguay,"[133] the estuary is, as a matter of fact, in a remarkable state of equilibrium, and there is no trace of a gradual accumulation of alluvia, or of important changes of channel. The shore of the delta north of the Parana de las Palmas, covered with rushes which protect it from the attack of the waves, shows neither advance nor retreat. The broad lines of the hydrography of the Rio de la Plata are plainly indicated on Woodbine Parish's map. The English Navy map of 1869 (on the basis of observations in 1833, 1844 and 1856) only differs in detail from the present map. The stability of the channels is surprisingly different from the changes in the bed of the river in the flood-zone. The permanence of the bottom, in spite of the loose deposits of the estuary, is explained by the regularity of the currents. These currents, which determine the submarine topography of the Rio de la Plata and the distribution of the banks, are not of river origin. They are tidal currents. [133] The water in the estuary, worked up by waves and tide, contains more sediment than the water of the river. There are two groups of shoals in the estuary. The first, the Playa Honda, occupies the whole western part of it up to a line drawn from Buenos Aires to Colonia. These banks leave a narrow passage in the north, opposite the Uruguayan shore, and this is followed by ships going to Uruguay and the Parana Guazu. The second group of shoals is the Ortiz Bank, triangular in shape, which rests in the north on the Uruguay coast below Colonia, while its point extends south-eastward to eighteen miles north of the Punta de las Piedras. It keeps the zone of deepest water in the middle estuary to the south, near the Argentine shore. In the latitude of the point of the Ortiz Bank, on a line from Montevideo to Punta de las Piedras, the middle estuary is separated from the outer harbour by a bar (_barra del Indio_) with thirty-eight feet of water, caused by the transverse currents which circulate from point to point inside the English Bank.