Page:Natural History of the Ground Squirrels of California.djvu/28

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In illustration of the fact of variability in depth and extent of burrow system with nature of soil, some actual instances as revealed, by excavation may be described. The layers of alkali hardpan in the Fresno region were found to have a very decided influence on the course of the burrows. In most cases where the hardpan was near the surface, the burrows were found to extend through the hardpan to the soft ground that is often to be found just beneath. No evidence was found to indicate that the squirrel had dug through even thin layers of solid hardpan except at points where natural cracks or openings through it occurred. Slight cracks in the hardpan were sometimes enlarged, this apparently having been done during wet weather, to sufficient size to enable the squirrel, but not such an enemy of the squirrel as a coyote or badger, to readily pass through. In following the various cracks and openings through and between the strata of hardpan, the burrows were found to twist about in very erratic fashion. The sudden elevation in a burrow of sometimes as much as two feet was found to form a very effective barrier to the flow of any gas such as that of carbon bisulphid, which is heavier than air; such a gas would gather into the low places (see Stewart and Burd, 1918).

The deepest burrow system uncovered was situated in an alluvial talus in the foothills near San Emigdio, Kern County. The maintenance of the great depth (from four to five and a half feet for a distance of twenty feet) was clearly due to the squirrel having followed a soft layer at the margin of the talus down to below the level of the four-foot rock-filled surface layer. Beneath this the squirrel had progressed easily through the soft soil as long as he kept beneath the rocks—which he was practically forced to do (see fig. 6).

There seems to be little or no evidence to support the rather widespread notion that ground squirrels burrow down until they reach water. A colony burrow was unearthed in an irrigated section near Bakersfleld, where the water level was known to be only five or six feet below the surface of the ground. No part of the burrow (see fig. 8) was found to extend deeper than four feet and hence not down to the water level. While ground squirrels do not absolutely require water, where surface water is to be had they often go considerable distances to secure it, going across the country sometimes as far as a quarter of a mile. In many places squirrels are found thriving where it is known that it is over 100 feet to ground water and miles to surface water.