Oceans of the Solar System - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 226

that fell on to the adjacent land, sweeping in eroded sediment and
recycling the water, for the cycle to begin anew. 130
Did such an ancient Martian ocean really exist? Or is it simply a
fanciful interpretation, akin to Percival Lowell's visions of Martian
canals? As the pictures of this enormous region grew ever more
detailed, not all of the evidence has pointed towards a former ocean. 131
The wide northern plains are not everywhere covered in fine sedi-
ment, as one might expect on an ocean floor far from land; large
boulders lie scattered about, here and there, and in some places there
are clusters of ridges and shallow depressions, while elsewhere there
are regions of jumbled and collapsed terrain. Further, at the margins
of the presumed ocean, there does not seem to be much in the way of
well-defined cliffs and stranded beaches. Some people therefore
thought that the northern plains had never been submerged under
deep water, but rather represent volcanic plains of ash and lava, while
another idea was that these flat areas had been 'resurfaced'—that is,
smoothed—by long-lived permafrost activity.
More recently, evidence in favour of a former Martian ocean has
once more turned up. A number of the canyon-fed deltas at the edge
of the southern highlands occur at close to the same elevation, sug-
gesting that—for a short time at least—this marked the position of a
shoreline. 132 The boulders scattered across the lowlands may have
been brought in by icebergs, 133 since any sea on Mars, so far from the
faint early Sun, would likely have been a frigid one, with a shifting and
fragmenting carapace of ice. Some of the linear depressions resemble
the kind of grooves that large icebergs can gouge on a sea floor.134 134
The areas of collapsed terrain also look strikingly similar to sonar
pictures taken of parts of Earth's deep-sea floor, where gigantic slabs
of waterlogged strata have slid down gentle slopes, breaking up as
they did so. 135 The balance of evidence seems to be tilting back in the
direction of some kind of former ocean, however short-lived. Most