When the amount of snow that falls in a region does not all melt during the year the accumulation may result in a permanent snowfield, a mass of ice, or a glacier. In the case of ice, the term glacier is not applied until the mass has reached the moving stage. The transition from snow to ice is brought about largely by the end of winter the usual snow bank is no longer composed of flakes or pellets of snow; instead it is a mass of granular ice to which the term neve is applied.

Where neve fields increase in thickness from year to year eventually the pressure compacts the lower portion of the mass into more or less solid ice; if the mass begins to move the name glacier is applied. Thus a glacier, at least at its source, has a stratification; snow overlies neve which in turn grades downward into more solid ice.

In Crater Lake National Park so little snow lasts through the summer under the present climate that solid ice usually does not form. However, the small amounts of snow that last through the summer as well as the snow that lingers until last June, July or August has been converted into neve. These patches of neve which last into or through the summer exert a limited though definitely noticeable weathering and erosional action.

On nearly level land the geological evidence of neve action (nivation) is perhaps most noticeable. Where neve lasts well into the summer, year after year, the site of the neve is lowered below its surroundings and a small depression is formed. Early in the summer season the accumulation of melt water at the base of the neve during the day is converted into ice at night only to be remelted the next day as more water trickles downward from the overlying neve. This repeated freezing and thawing acomminutes the rock particles. Some water drains downward through the mantle and out of the depression and carries away the finer rock particles. In this manner the depression is enlarged and deepened by the same process of nivation that inaugurated it.

On a sloping terrain nivation often is more active although its evidence may be difficult to distinguish from that of normal erosion. Its results may resemble those of slides and creep phenomena. As nivation continues on a slope the resulting concavity or niche approximates a cirque in appearance although hardly in size. On bedrock nivation operates more slowly than on mantle although the results are similar.

In the higher portions of Crater Lake National Park, nivation is an important and evident geologic process. In the forests and at the base of the talus slopes within the caldera the evidence is not so apparent, but on the treeless expanses there exist many noticeable areas. Several representative examples border the highway from Park Headquarters in the Rim Village near its upper end. These rather flat, treeless expanses are concave upward as a result of greatest activity near the center of the neve field, which in 1949 lasted well into July.

On the back slope of Llao Rock, numerous areas of nivation are easily identified. Some are occupied by neve so late into the summer that practically no vegetation occupies them although along their margins soil and grass cover the pumice at the edge of active nivation. At several localities small serpentine ridges (of) dust-like material were observed on melting of the neve. These ridges, two to four inches high, were also traceable under the neve and marked the egress of streams or rivulets of melt water. Although it is known that rodents dig trails under the snow and neve these were not burrows near the observed ridges. Instead the tiny ridges were composed of water-carried and water-deposited material. In effect they were eskers on a very minute scale.

Numerous forest-free slopes on the higher elevations in the park are the sites of active nivation. Downwards these sites grade into areas which posses similar appearing characteristics and which are believed to have been subject to nivation during past periods of heavier snowfall. On steeper slopes facing both inward and outward in respect to the caldera, nivation is believed to be an active geological agency whose results are largely obscured or exceeded by creep and slide. The small ridges of water-deposited silt are identified as esker-like features produced by sub-neve runoff.