1908.] 



SEASOXAL PHENOMENA : 



-MACKENZIE VALLEY. 



39 



inches, but the excavation was not carried to a sufficient depth to reach 

 tlie permanently frozen substratum/' 



On March 26 the temperature first rose above tlie freezing point and 

 from tliat date did not descend below zero. AMien the spring thaw set 

 in the snow had attained a depth of nearly 4 feet. 



« In this connection it may be well to present some additional data rejiarding 

 ground ice at Fort Simpson. The information was given me by A. F. Camsell, 

 of that post. 



In excavating a cellar on a sandy ridge in the midst of a field on .July 21, 

 1903, frozen ground was reached at a depth of 7 feet. In October, 3903, an 

 excavation made in the yard of the dwelling house revealed frozen ground at 

 7 feet. Recent frosts had penetrated about 4 inches from the surface. This 

 yard was sheltered by a high, tight fence. 



In October, 1901, a pit was sunk beneath an engine house which had con- 

 tained a fire during two previous winters, and frozen ground was encountered 

 at a depth of 25 feet. A foot of frost was succeeded by a foot of thawed 

 ground, beneath which frozen ground was again reached and was penetrated 

 a short distance. Two years later the hole was deepened 3 feet through 

 ground which had thawed since the pit was originally dug, and frozen ground 

 was again encountered. It is proper to state that this pit was situated only a 

 few feet from the edge of the river bank, where the effect of the summer heat 

 would be greater than in a situation where the soil was not exposed to this 

 lateral influence. 



Relating to the same phenomenon at Fort Simpson, Richardson says : " In 

 October 1836, a pit sunk by Mr. M'Pherson, in a heavy mixture of sand and 

 clay, to the depth of 16 feet 10 inches, revealed 10 feet 7 inches of thawed soil 

 on the surface, and 6 feet 3 inches of a permanently frozen layer, beneath which 

 the ground was not frozen. (Arctic Searching Expedition. I, p. 166, 1851.) 



Permanently frozen ground occurs in many parts of the north. According to 

 some authorities, its southern limit is the isotherm of 32°. It is unfortunate 

 Ihat more observations regarding this phenomenon, especially as regards the 

 thickness of the frozen substratum, have not been recorded. There is reason 

 to believe that in muskegs and marshes the summer thaw penetrates to a 

 shorter distance than in dry ground. On .June 3, 1901, in a marshy pond near 

 Fort Chipewyan, which had been free from ice for about a month, the muddy 

 bottom was still frozen solidly. In the middle of June. 1901, the bottom of a 

 muskeg at Fort Norman flooded with a foot of water was still solid. In such 

 situations it is likely that the covering of cold water prevents the summer 

 heat from penetrating to any considerable depth. 



McConnell contributes the following : "Around Great Slave Lake the soil 

 seldom thaws out to a greater depth than eight feet, and in many of the muskegs 

 and marshes ice remains throughout the year at a depth of about two feet. In 

 descending the Mackenzie the frozen soil gradually approaches the surface. At 

 Fort Norman at the end of summer it lies at a depth of about six feet, at Fort 

 Good Hope at about four feet, and at the mouth of Peel River at about two feet. 

 The thickness of the frozen stratum was not ascertained." (Ann. Rept. Can. 

 Geol. Surv., IV, p. 32 D, 189L) 



Richardson states that in the vicinity of Fort Franklin the soil during the 

 greater part of the year is firmlj- frozen, the thaw in the two seasons 

 [1825-26] we remained there never penetrating more than twenty-one inches, 

 from the surface of the earth." (Franklin's Narr. Second Expedition to Polar 

 Sea, Appendix, p. XI, 1828.) 



