The formation of hummocky ground on a morainic hillside appears to be somewhat as follows:—
If conditions, such as snow-line, wind, etc. (on a bare hillside of sandy and stony ground) permit, isolated plants begin to appear, and collect soil around them by their prostrate habit of growth and with their roots; in consequence of the loose nature of the soil, and owing to the action of snow or water, slipping frequently takes place, and the plants tend to keep up the earth. As the plants spread, terraces get formed and the plants bank up the loose earth, and the mountain-side is then broken up into broad steps. The vegetation continues to grow over the edges of the terraces, and in consequence more soil is collected by the plants, and small mounds result, and later on, if the slope is not too great, true hummocks may be formed. If the slope is steep, the hummocks are usually flat-topped, but this is also often due to snow, wind, etc.
The hummocky ground usually seen occurs in fairly level country, and its origin may probably have been somewhat different from that just described; the hummocks are from one to two feet high, and the ruts between are often so narrow that two hummocks have frequently united. The normal breadth of a hummock is from one to two feet across, and it is closely covered by Empetrum or Cassiope, with Carex vulgaris, the Moss Rhacomitrium lanuginosum, and the other plants already enumerated.
Hummocky ground was also met with in meadows at Gilhagi (near Maelifell); here they were covered by grass and other meadow-plants, and Saxifraga cernua grew in masses in the depressions. These hummocks were on a hillside, and may have been originally formed in the manner already described; but owing to some change in the snow-line, meadow-land plants may have wandered up the hillside, and ousted the characteristic plants of the hummocks.
Our journey from Thjofadal to Gránanes lay for the most part over the lava-flows from Strytur, which were covered by a close-growing vegetation, large tussocks of moss, very dwarf Saxifrages, S. cæspitosa, Empetrum, Salix herbacea, and other plants usually found on the hummocks. True hummocks were, however, rare.
Near our camp and before the Svatá was reached we came to the end of the lava and crossed a sandy tract, and the usual change in the flora was at once noticeable; large clumps of willows bound the sand together, around which the soil collected, forming mounds, and patches of Armeria, Arabia petræa, Silene, and Arenaria were scattered about. In some places the willows were killed by having been buried in the blown sand.
Between Gránanes and Hvitarvatn the country is at first hummocky, and then covered by birch and willow scrub. On the hummocks, especially in the patches of moss, Saxifraga Hirculus was not uncommon. The large tract of swampy ground at the edge of the lake was covered by masses of cotton grass. Leaving Hvitarvatn our way lay through willow scrub until the Hvitá was reached. After crossing the river the ground was very rough, being composed of morainic matter from the adjacent mountains and covered with large angular blocks of lava. The only flowers growing here were Arenuria, Armeria, Thymus and Dryas in isolated patches. As we travelled farther south, the plants were noticed to be taller and more vigorous than those seen in the interior, and the flora was more home-like in character. Juniperus, Betula odorata, Calluna, and Arctostaphylos uva ursi were noticed for the first time.
Both birches (B. odorata, B. nana) were very plentiful, and from two to three feet high (in the north B. nana was only six to nine inches high) willows were not common.
Between Sandá and Gullfoss another small desert area was traversed where earth-pillars occurred. The plants were hardly able to exist, as the smaller ones were blown away, and the willows and birches were in many places buried by the blown sand.
Around Gullfoss the vegetation was luxuriant, Geum rivale, Geranium sylvaticum, and Alchemilla vulgaris covered the ground, and above Rubus saxatilis and Frugaria vesca were abundant, whilst Arctostaphylos uva ursi also grew in profusion.