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Walther Penck (1888–1923) came up with a theory on landscape development which was totally different from the geographical cycle of erosion by W. M. Davis. Penck believed that instead of just passing through a set of stages, the landform development depends on how tectonic uplift and denudation happen simultaneously.

• Penck’s concept of time
  Penck did not believe in the notion of landscape going through the succession of youth → maturity → old age.
  Penck suggests:
  Uplift and erosion happen together.
  The process of landscape evolution is continuous, and there are no well-defined stages.
  The slope shape is a function of the uplift rate and the denudation rate.
  Consequently, the evolution stage is not influenced by time only.
  Different areas of the same landscape might evolve in different ways since the different uplifting and erosion processes affect changes.
• Penck’s three developmental phases
  Penck relates the process of landform development to different uptakes of the crustal layer:
  3 phases: condition of uplift process of uplifting
  Waxing development increasing uplift rate convex slopes
  Uniform development constant uplift rate straight slopes
  Waning development decreasing uplift rate concave slopes
• The significance of time
  The fundamental distinction from Davis is:
• According to Davis:
  
  Time → period of erosion → landscape form.
  According to Penck:
• 
  Combination uplift+denudation+time → landscape form.
  Hence, Penck considered time to be a constant reference frame in which tectonic movements and denudational processes take place simultaneously.
  Penck has never introduced fixed chronological periods of evolution such as youth, maturity, and old age; but rather focused on the ratio of the crust movement and denudation.

Readings and Sources:

Geomorphology: Progress With Time

Principle of Uniformitarianism : A Basic Principle of Geomorphology

Role of Geomorphology in Engineering Project