Rock weathering, or the mechanical and chemical breakdown of rock over time, creates the landscape on which all terrestrial life is built. Weathering produces sediment, allows the carbon cycle to proceed, and provides a rich soil substrate on which organisms can grow, die, and decompose. The cyclicity of Earth processes is a function of weathering at all scales. The primary purpose of this Dissertation is to quantify the rates and controls over mechanical weathering [rock cracking/fracturing] of surficial boulder deposits in Eastern California using field data collection, laboratory analysis, and simple mathematical modeling. I collected rock and crack field measurements, clast size distribution data from the field, and rock elastic properties through laboratory testing. I used a chronosequence or space-for-time approach, a method often employed by soil scientists [Jenny, 1948; Birkeland, 1999], whereby data are collected from rocks or sediments that have been exposed to natural weathering conditions for a range of times, using the properties of the stable deposits to represent the amount of weathering that occurs over the time span of exposure. I studied rocks at three sites, within which each site's climate and rock types are the same, but rocks have been deposited starting 148,000 years ago and continuing into the present with active channels and washes. In the field, I manually measured 8763 crack lengths, widths, and orientations from 2221 in situ boulders on Earth's surface. These data were processed in JMP and Excel software, with some interpretation of data performed using Python and MATLAB. In Chapter 2, the data show that rock cracking is initially fastest when rocks are exposed to Earth's surface conditions and begin to weather, with rocks accumulating cracks at a rate of 9-1502 mm of cracks per m^2 rock surface over a thousand years, or 0.1-36 individual cracks per m^2 rock surface over a thousand years. After this point, rocks continue to crack, but the rate of crack growth slows down. After about 30,000 years [30 ka], the growth rate is <36 mm of cracks/m^2 of rock surface per ka, or <1 individual cracks/m^2 of rock surface per ka. From two of these sites, I also collected a single granitic boulder from each dated deposit, and performed standard rock mechanics testing [Chapter 3]. These data show that rock compliance increases over time while mechanical weathering leads to an increase in microscale cracks, which do not lead to the rock breaking into pieces, but effectively alter its strength and elastic strain response under stress.Using the laboratory analyses and local weather station data, I implemented a simple daily stress model that uses Paris' law of subcritical crack growth to predict single crack growth after each day of weather conditions. I extrapolated the weather data out to 5000 years to determine whether this simple model can predict the cracking observed in the field. The magnitude of cracking itself was the slowest at the coolest, semi-arid site, then was faster at the two warmer sites to the south. Cracking occurred over only a limited number of unusually intense weather days [Ch. 4] when the daily range of air temperatures [air temperature flux] was the largest. In the two semi-arid sites, these cracking days were hot, dry summer days; in the arid site, the day when the most crack growth was predicted coincided with summer monsoonal rains. The model is highly sensitive to rock elastic properties, which supports the theory that a gradual increase in bulk compliance [Ch. 3] allows rocks to withstand stress without cracking over thousands of years [Ch. 2]. To better understand the drivers of cracking I performed statistical comparisons among rock and crack field data [Ch. 5], and determined that age itself has the most consistent, positive, statistically significant correlation with the number of fractures per rock surface area [fracture number density] and the total length of fractures measured per rock surface area [fracture intensity]. This suggests that counting cracks on rocks is another simple and reliable metric for relative age dating in the field. Other factors like lichen growth and varnish development increased with rock exposure age and generally acted to infill and decrease the number or length of measurable cracks on the rock. Lithology was also an important factor, but even with this large dataset, more advanced bulk rock modeling and/or more precise compositional and grain size data than the categorical indices employed herein, are required.Finally, I present clast size data to show that rocks decrease in overall size with time, and that the fastest cracking upon initial exposure [Ch. 2] corresponds with the abrupt increase in small clasts that has long been recognized in the context of the development of desert pavements. For volcanic and carbonate rocks, there is a correlation between the geometry of cracking observed on the rocks and the shape of sediments on older deposits: when many cracks are parallel to the rock surface, older deposits tend to have more flattened rocks on them. This shows that cracking rates and crack geometries can play a strong role in clast size and shape evolution over geologic time, and mechanical weathering should be considered when interpreting sediments in the geologic record.Overall, I find that rock cracking rates decrease over time [Ch. 2], during which time rock mechanical properties like porosity and permeability increase, and rock density, strength, and Young's modulus [incompressibility] decrease [Ch. 3]. These findings are directly applicable to geoscientists attempting to understand sediment production from larger pieces of rock, and help constrain the geochemical reaction rates that drive the carbon cycle. Rock fall and landslide hazards can also be better assessed through this understanding of progressive rock cracking, its controls, and its environmental drivers.More broadly, the decreasing rock cracking rates that accompany slow mechanical property changes represent a real-world example of material fatigue vs. material failure. Engineers and geologists concerned about present-day Earth conditions must understand that what they see now is not necessarily what exists underground, and that rock properties are evolving over timescales beyond direct human observation. As geologists commonly employ a uniformitarianism approach [Hutton, 1899] whereby what we see happening now is presumably representative of what has happened in the past, it is critical that all geologists understand how dramatically rock physical properties can change once the rock reaches the Critical Zone. This Dissertation offers an analysis of the slow, but critical, process of Earth surface mechanical evolution.