The production of regolith is a fundamental geomorphic process because most
surface processes transport only unconsolidated material. We use concentra
tions of the cosmogenic radionuclides (CRNs) Be-10 and Al-26 in regolith an
d bedrock to deduce the rate of production of regolith on an alpine hillslo
pe in the Wind River Range, WY. These calculations are based on a theoretic
al model which we develop here. This model shows that it is important to co
nsider dissolution of regolith in regolith production and in basin-averaged
erosion rate studies. Rates of production of regolith are uniform along th
e hillslope and the mean rates for the entire hillslope deduced from Be-10
and Al-26 are 14.3 +/- 4.0 and 13.0 +/- 4.0 m Ma(-1), respectively. Rates o
f production of regolith deduced from Be-10 concentrations in regolith-mant
led bedrock support the rates deduced from regolith concentrations. In the
alpine environment examined here, the rate of production of regolith beneat
h similar to 90 cm of regolith is nearly twice as fast as the average rate
of production of regolith on bare rock surfaces, which Small et al. [Small,
E.E., Anderson, R.S., Repka, J.L., Finkel, R., 1997. Erosion rates of alpi
ne bedrock summit surfaces deduced from in situ Be-10 and Al-26. Earth and
Planetary Science Letters 150, 413-425] previously documented. Rock-mantled
with regolith probably weathers more rapidly than bare rock because the wa
ter required for frost weathering is limited on bare rock surfaces. Because
the hillslope examined here is convex with constant curvature and regolith
production and thickness are uniform down the slope, the regolith volume f
lux must be proportional to the local slope of the hillside. Therefore, our
results are consistent with Gilbert's [Gilbert, G.K., 1909. The convexity
of hilltops. Journal of Geology 17, 344-350] steady state hillslope hypothe
sis. If tor height and the difference between rates of weathering on bare a
nd regolith-mantled rock provide a fair estimate of the age of summit flats
, steady-state hillslope conditions have been attained in less than several
million years. (C) 1999 Elsevier Science B.V. All rights reserved.