152 U. S. NATIONAL MUSEUM BULLETIN 217 



but the tree line is low (1,600 feet) and the forest rather marginal 

 along the Old Crow. 



At the relatively low elevations of the headwater valleys north of 

 Old Crow occasional spruce extends to the headwaters of the Firth 

 River, flowing toward the arctic sea. This is about the northernmost 

 occurrence of timber in America. The broad valley east of Old Crow 

 Eiver and north of the Porcupine is occupied by many lakes in Crow 

 Flats, which occupy a depression among the mountains in arctic 

 Yukon east of the high peaks of the Brooks Eange in northeastern 

 Alaska. From Crow Flats, however, only little drainage passes 

 through Old Crow River after the spring flood, for the annual pre- 

 cipitation is evidently small. Between the Yukon arctic coast and 

 Crow Flats the narrow Range of the Richardson Mountains extends 

 the Brooks Range eastward along the arctic coast and continues south- 

 ward along the Yukon-Mackenzie border. 



The Porcupine River which rises in the northern watershed of the 

 Ogilvie Mountains and runs 150 miles northeasterly before changing 

 its course westward near the mouth of Bell River, has its southeastern 

 headwaters not far from those of Peel River. On the map, the 

 passage of the Peel through the southern Richardson Moimtains ap- 

 pears to provide a valley route at low elevation by which birds might 

 enter the upper Porcupine Valley from the Mackenzie, while south- 

 ward and southwesterly the Ogilvie Range interposes what looks like 

 a barrier of high and extensive mountains separating the upper Por- 

 cupine Valley from the upper sources of the Yukon. The route to 

 northern Yukon Territory which seems to be open for birds migrating 

 at low elevations from the Alaskan part of the Yukon Valley extends 

 northeasterly along the Porcupine. The Porcupine Valley in Yukon 

 lies north of the Rocky Mountains in some isolation by surrounding 

 ranges through which, however, it provides lowland connection be- 

 tween the interior valley of Alaska and the Mackenzie Valley. 



Warm weather comes to Old Crow a few weeks later than to Fort 

 Yukon and summer is colder. About June 25, 8 inches of snow brought 

 toboggans into use at Old Crow for two days. Since frost usually 

 occurs in every summer month there is little chance for gardens. The 

 summer weather at Old Crow is more severe than at Bettles, but is 

 probably not quite so cool as Anaktuvuk. Old Crow is evidently 

 just within the western arctic region where the close spacing of iso- 

 thermal lines (see fig. 2) shows the rapid northward cooling of summer 

 temperatures. 



Of special interest in regard to the present geographical distribution 

 of life in Alaska and Yukon is the history of the recent glaciation, 

 which has left many clear marks, and the related climatic history, 

 which is less obvious. In Wisconsin times Cordilleran ice (Flint, 



