165 



A Probable Origin of the Numerous Depressions in the 



Mesa South oe the Arroyo Formed by the Outlet 



of TuERAS Canyon in the Sandias Near 



Albuqueeque, New Mexico. 



By Albert B. Reagan. 



The occurrence of numerous slight depressions, thickly distributed 

 over the mesa south of Tijeras arroyo on the east side of the Rio Grande, 

 south of Albuquerque, New Mexico, is very noticeable. These were ob- 

 served to be rarely more than five yards across and commonly from eighteen 

 inches to two feet in depth and are provided with a raised border. They 

 resemble buffalo wallows very much ; but are too abundant and their dis- 

 tribution is too general. The stratum in which they are indented is a very 

 loose, unlithified formation, superimposed upon the Albuquerque Marl,* a 

 calcareous deposit some six feet in thickness. 



The depressions extend in depth to this marl stratum and seem to hold 

 water. 



These depressions seem to be the "blowouts" of mud upheavels. They 

 seem to have been formed at the time the Albuquerque marl was in a semi- 

 fluid state. The loose unlithified stratum that is superimposed upon the 

 marl was washed down from the Sandias onto the area faster than the 

 marl could harden or "ereep" from its advance over the bottom of the 

 then Albuquerque lake which occupied the Rio Grande embayment at that 

 point. As a result of the pressure caused by the superincumbent weight, 

 mud lumps formed in size proportionate to the pressure, like those now 

 forming in the Mississippi Delta.** And like those of the Southeast Pass 

 of that delta, they collapsed on reaching the mature stage, leaving a small 

 pit surrounded by a raised ring. Thus the mud lump, "blowout" theory 

 seems to explain the origin of the depressions. 



*C. L. Herriek. The Geology of the Environs of Albuquerque, New Mexico. Amer- 

 ican Geologist, Vol. XXII, pp. 29-33. 



**E. W. Hilgard. The Exceptional Nature and Genesis of the Mississippi Delta. 

 Science, Vol. xxiv, pp. 861-866. 



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