We exploit exogenous bank deposit windfalls from oil and natural gas shale discoveries to demonstrate the importance of bank branch networks in integrating the U.S. mortgage market. Using loan level data we find that banks exposed to shale booms increase their lending in nonboom counties by 0.93% per 1% increase in deposits. This effect is present only for bank lending via local branches and is strongest for mortgages that are hard to securitize. Our findings suggest that agency and information frictions limit the ability of arm’s length finance to integrate credit markets. To our knowledge, these results provide the first ‘smoking gun’ evidence that branch networks play an important role in financial integration, despite the development of securitization markets and advances in lending technology.